Why is “wavelength” one word when “wave height” isn't?

As another example, wave speed is two words. But wavelength is only one word. What is the reason for this?

In Swedish and other contructs, both words are only one word:

  • våglängd (wavelength)
  • våghöjd (wave height)
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    Wavelength has been adopted as a unit of measurement. The other phrases have their respective terms, (wave height: amplitude) as such they are separate individual words.
    – Kris
    Commented Oct 23, 2012 at 12:00
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    Also, wavelength is used in multiple disciplines and so, being more frequently used, it became spelled solid as phrases are wont to do. See this NGram of wavelength, wave height, wave length,
    – JLG
    Commented Oct 23, 2012 at 12:09
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    Post-edit comment: The case of Swedish, and even German, could be different from English: Many adjective+noun phrases/ phrasal nouns are single words in German, combining even three or more 'words'. It's very rare in English.
    – Kris
    Commented Oct 26, 2012 at 12:26
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    If the underlying question is really why it is that Swedish and other languages form compounds much more frequently than English, I suggest migrating your question to Linguistics.SE.
    – MetaEd
    Commented Oct 26, 2012 at 15:06
  • Both wave height and wave speed are compounds. The only difference is that someone happened to decide to write wavelength closed up, while the same never happened with the others. Most compounds in English are written as separate words, unlike other Germanic languages; that doesn't make them any less compound. Writing does not accurately represent actual linguistics fact. Commented Mar 13, 2014 at 8:05

5 Answers 5


The reason that wavelength is only one word is that humans commonly form new words by combining existing words together. This is called compounding, and is observed over and over by linguists studying the evolution of languages.

The reason that the other terms you mention, “wave height” and “wave speed”, have not compounded is that they are different from “wave length” in a significant way.

Online Etymology Dictionary tells us that the compound wavelength was coined in the 1840s when scientists began to explore the properties of light spectra (rainbow colors). As Kaz’s answer points out, they needed a technical term for the “distance over which the wave’s shape repeats”. (Wikipedia)

However, a new word was not coined for light’s “wave height” or intensity. Instead, the much older existing term amplitude was used. Nor was “wave speed” used for light’s velocity: instead the term speed of light was adopted.

Those choices are just an accident of history. But the consequence is that “wave height” and ”wave speed” did not gain currency in the 19th century explosion of literature on the subject of electromagnetic waves.

We can actually see compounding occurring in the chart below. Three similar terms – wave length (purple), wave-length (green), and wavelength (blue) – rapidly gained currency as scientific inquiry progressed. We see that before 1920 the term was almost always spelled using a hyphen or space. During the next three decades, the compound spelling wavelength gradually displaced the other spellings. By 1950 the majority of authors used the compound spelling.


  • "Online Etymology Dictionary tells us that the compound wavelength was coined in the 1840s when scientists began to explore the properties of light spectra (rainbow colors).": The other compounds were not coined. Which, in short, is what my answer says. On ELU, we are trying not to get too much into technicalities.
    – Kris
    Commented Oct 25, 2012 at 3:40
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    I don't think that word we means what you think it means.
    – MetaEd
    Commented Oct 25, 2012 at 15:18

Wavelength has been adopted as a term for a dimension (of measurement).

The other phrases have their respective terms (wave height: amplitude), as such they are separate individual words.

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    Actually "wave height" is twice the amplitude (en.wikipedia.org/wiki/…), but the answer is still true as wavelength is a unit of measurement and wave height is not.
    – Nir Levy
    Commented Oct 23, 2012 at 13:44
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    @NirLevy I agree. It was not intended that wave height equals amplitude, though. I should have been more specific.
    – Kris
    Commented Oct 23, 2012 at 14:08
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    Not so much a "unit" of measurement as a dimension of measurement. We don't measure things in wavelengths; we measure wavelength in units of length (usually based on observed time from peak to peak, and known wave speed)
    – KeithS
    Commented Oct 23, 2012 at 14:42
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    @KeithS We do measure some things in wavelengths: for example, organ pipes. You will see the length of the pipe expressed as a fraction such as 1/2 of the length of the standing wave. However I do not think the fact that wavelength can be used as a measurement has anything to do with why it was compounded. See my answer.
    – MetaEd
    Commented Oct 23, 2012 at 16:14
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    Rf circuits are also measured in wavelengths, or in degrees where each degree is 1/360th of a wavelength. Commented Oct 23, 2012 at 22:30

Note that wavelength is a noncompositional compound. It does not refer to the length of a wave, but rather to the length of a single oscillation period of the wave: the reciprocal of the frequency. It is technical jargon which does not mean wave length.

Generally we write two words as one when they are used so often and in such a specific way, that the meaning is no longer just the combination of the meanings of the two words. For example, a blackboard is not a black board. It is not just any board which is black but a tablet for writing with chalk, which may in fact be produced in colors other than black, such as green. Moreover, this is not just an orthographic (written) difference. The word blackboard is phonologically different from black board. It places accent on black and removes it from board. There can be an accent on black in black board, but it is not integrated into the word. It is used for emphasis, as in "I said bring me the black board, not the white one!". The accent could also be on board: *"I said bring me the black board, not the black cord.". In blackboard, there is no freedom to play with the relative stress. An example which shows that this has nothing to do with orthography is (The) White House, which is is written as two capitalized words, yet it is phonetically a unit like blackboard.

Waveheight has not been coined as a technical term probably because English-speaking scientists and engineers adopted the term amplitude instead, which is as precise as wavelength. Amplitude is the degree of displacement of an oscillation from the system's rest position.

If waveheight referred to amplitude, it would be different from wave height, which does not have a clear meaning. Wave height could refer to amplitude, or it could be a peak-to-peak height, which is twice the amplitude. Or it could be something else: the height of shoaling as water waves hit shallow water.

  • "Wave height" does not have a clear meaning only because we have not established it. There is no logical reason "wave height" could not be as precise as "amplitude". I also disagree that amplitude is as precise as wavelength. Depending on the shape of the wave, the wavelength could be completely obvious and unambiguous while the amplitude is not. There are different definitions for "amplitude", just as there are different definitions for "average".
    – John Y
    Commented Oct 23, 2012 at 21:08
  • We are simply not free to establish a meaning for wave height such that we can dictate this meaning to all English speakers in all situations, because we do not control the rules by which speakers combine nouns to make compound nouns.
    – Kaz
    Commented Oct 23, 2012 at 21:22
  • We could coin the word waveheight, give it a specific meaning, and try to popularize it. But that has no effect on wave height. It is a different word which is not only written as a unit, but exhibits different phonology. There is a stress on wave, but not on height. Whereas in wave height, the stress is more or less equal.
    – Kaz
    Commented Oct 23, 2012 at 21:25
  • I'm not saying we (you and I) are free to establish anything on our own. I'm saying that the English-speaking population at large has (for various reasons, including pure chance) not established a more formal definition for wave height. To your other point, there is no reason a two-word term cannot have absolute precision and scientific rigor. We have such a thing as atomic mass, but no atomicmass or atommass, for example.
    – John Y
    Commented Oct 24, 2012 at 4:51

The only full answer is “because it is”. There are rarely complete answers to “why” questions in linguistics.

But a clearly relevant factor is that there is a need to talk about wavelength much more often than wave height, because wavelength is a property of all kinds of wave (including sound and electromagnetic radiation), whereas height is a property only of transverse waves in a physical medium, such as water.

  • Colin, wave height is a property of many types of waves. God bless Wikipedia for explaining this to me.
    – Nir Levy
    Commented Oct 23, 2012 at 13:47
  • A wave that has wavelength always has wave height as well, 1:1. :)
    – Kris
    Commented Oct 23, 2012 at 14:14
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    @Kris Nope, not every kind of wave. Ordinary sound for instance has a wavelength but not a height.
    – Mr Lister
    Commented Oct 23, 2012 at 19:28
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    Yes, 'because it is' seems the only answer to the question 'Why is particleboard spelt particleboard ... or particle-board ...or particle board?' Commented Oct 23, 2012 at 20:06
  • @MrLister (and Colin): In a very literal sense, I suppose it's true that not all waves have height. But anything that can reasonably be called a wave has some kind of amplitude, and for visualization, general discourse, and even bona fide physics, we trivially map amplitude to the transverse-wave concept of wave height. Granted, these terms are less nailed down than "wavelength" (e.g. peak vs. RMS amplitude, etc.), but they are no less intrinsic to waves.
    – John Y
    Commented Oct 23, 2012 at 20:48

The real answer to this question, in my opinion, is that to use the word height to describe a wave displays both a misunderstanding of the actual physical form of many waves and an incomplete understanding of the word height as it relates to mathematics.

First, most people think of waves in terms of the surface of water, or the readout of an oscilloscope, however many waves are not physically shaped like that.

Second, mathematically, height is the third dimension of measurement and it is perpendicular to the plane created by length and width. While a wave travels three-dimensionally, it doesn't actually have that kind of dimensionality, a wave as we measure it is shown to have dimensionality, because we prefer to perceive things as we are, not as they are, but a wave as it is, is actually just energy travelling.

As a quick aside, when I try to think of a wave as it is, I always come back to the word impulse, I kind of feel like that it the closest word we have to describe waves-as-energy-transfer that relates to an identifiable feeling we experience.

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