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For instance, biologists these days like to say that the word "dinosaur" is inclusive of modern birds, since birds are descended from dinosaurs. This is consistant with biologists' tendency to categorize things monophyletically -- that is, if it is descended from something, it "is" that something. A bird is descended from a dinosaur, so it must be a dinosaur.

Linguists (such as those employed by dictionaries e.g. Webster and Oxford) tend to be more pragmatic -- concerned more about communication and usefulness rather than strictly adhering to monophyletic categories. They consider "dinosaur" to only refer to those animals that lived in the mesozoic period....that is, they explicitly exclude modern birds from the definition, because when most people use the term "dinosaur," they are not talking about birds.

This is similar to the way we exclude tetrapods (amphibians, reptiles and mammals) from the definition of "fish", even though tetrapods are descended from fish. That is, "fish" is, by definition, a paraphyletic category. Other examples are "monkey" and "ape", which, by definition, are paraphyletic and therefore exclude humans.

So who is the authority on the definition of these words? And if biologists are, isn't it being inconsistant to define the word "fish" to exclude a subset of its decendants, while refusing to do the same with the word "dinosaur"?

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closed as primarily opinion-based by tchrist, Andrew Leach, JSBձոգչ, GEdgar, choster Jun 26 '13 at 16:08

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise.If this question can be reworded to fit the rules in the help center, please edit the question.

Who says there has to be one single authority? In the kitchen, cucumbers are not fruit. – congusbongus Jun 26 '13 at 5:19
And I bet not many scientists would look for them in the fruit section of the local supermarket. People readily use different registers - otherwise confusion and frustration would reign. – Edwin Ashworth Jun 26 '13 at 8:51
Linguists describe, teachers prescribe, scientists stipulate, that kid in the back makes a fart noise. – Mitch Jun 26 '13 at 11:51
Tell me who said I am an ape. – Kris Jun 26 '13 at 13:25
I was rather shocked to find A whale is a big fish in what looks like a current "educational" book where the child is supposed to write Yes or No against each statement. And presumably be marked down for putting No there. Is it any wonder Creationism still attracts people, when that's what they're being taught at the elementary level before they even have any critical faculties? – FumbleFingers Jun 29 '13 at 0:33
up vote 4 down vote accepted

It is you and I, the everyday users of the living language, who unwittingly "define" the contemporary semantics of a term.

Usage varies with time, with geography and with context.

One thing to remember is that scientific definitions are comparatively static -- they stay rigidly far, far longer than those in colloquial language, for a purpose -- so scientists can communicate without ambiguity.

Dictionaries try to resolve the dichotomy to some extent by labeling the terms as 'technical', 'formal', 'informal', 'colloquial', 'slang' and so on.

di·no·saur /ˈdīnəˌsôr/ Noun
For a scientist,

A fossil reptile of the Mesozoic era, often reaching an enormous size. …

For us, mere mortals, it sometimes also means

A person or thing that is outdated or has become obsolete because of failure to adapt to changing circumstances.

I don't know if the word really means 'bird' for anyone, except in the appropriate technical context.

The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic Period and, consequently, they are considered a subgroup of dinosaurs by many paleontologists. (WP) [emphasis mine]

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Taxonomists are those who are "in charge" of the categorization of such things.

Linguists are in charge of determining and noting current usage, which may be at odds with taxonomy. Dictionary editors often note this. For example, note the definition of "bug" in an unabridged dictionary.

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"Determining usage" is the converse of "defining the meaning," which the question is about. – Kris Jun 26 '13 at 13:52

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