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I haven't found a ancient Greek site on stack exchange, so i hope it is ok to ask it here: What is the difference between 'iso-' and 'homo-'? Do they both mean 'same'?

For example: isotope, isomer, isostasy, isomorphism... homophone, homosexuality, homomorphism...

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    iso from isos (equal) homo from homos (same).
    – Jim
    Commented Oct 14, 2013 at 0:19
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    The meanings are not quite the same. And there is very little scope for replacing one prefix with the other in a given word. In maths, isomorphisms are a special type of homomorphisms. Since 'sameness' (identity) seems a more rigorous constraint than 'equality', one might expect the relationship to be the other way round.
    – Edwin Ashworth
    Commented Oct 14, 2013 at 0:31
  • @Jim: unfortunately I don't quite know the difference between 'same' and 'equal' in most contexes. In german it's both 'gleich', so maybe I wrongly percieve the seemingly subtle differency as abit blurry. Could you please make me an example?
    – Matthaeus
    Commented Oct 14, 2013 at 0:49
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    @Matthaeus- Consider denominations of money, two 50 unit pieces is equal to one 100 unit piece, But they are not quite the same: 2 coins vs 1 coin or 2 coins vs 1 paper bill.
    – Jim
    Commented Oct 14, 2013 at 1:19
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    So here in a Language group, we must not confuse homonyms with isonyms...
    – GEdgar
    Commented Oct 14, 2013 at 13:35

3 Answers 3

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It is the difference between two things being equivalent or identical. Homo- means identical and iso- means equal or equivalent.

There is a subtle but clear difference between equivalent and identical. For example, the prime minister of the UK is the equivalent of the chancellor of Germany. In both cases, the person holding office is the head of the government. The two are equivalent, but they are not identical.

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  • Of course, we're focusing on the principal senses of the words / prefixes here. Words tend to broaden in meaning – identical twins aren't identical (one ate more aubergine last week – becoming broader). They may perform equally well on a maths test one week, but ... . Which makes it rather surprising (at least to me) that there aren't any? many? doublets of the forms isowhatsit and homowhatsit.
    – Edwin Ashworth
    Commented Oct 14, 2013 at 10:44
  • @EdwinAshworth yes, I tried to find some easy examples and was stumped. You do get some in scientific/technical vocabulary but they are not commonly used outside their fields.
    – terdon
    Commented Oct 14, 2013 at 13:35
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    @EdwinAshworth: the only one i know of is isomorphism vs. homomorphism. But stem words with iso- as prefix seem to have quite often also variants with 'homeo-' as prefix (e.g. homeostatic, isostatic) even though, that's something else completely
    – Matthaeus
    Commented Oct 17, 2013 at 3:17
  • Yeah, I there are at least too many exceptions for this distinction outside of strictly defined technical vocabulary (isomorphism vs homomorphism). Out of OPs examples even, homophone == exactly same sound, homosexuality == exact same sex (supposing binary sexes :-)).
    – Ciro Santilli OurBigBook.com
    Commented Jun 20, 2020 at 21:19
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Some interesting views, on reddit

"Homo-" means "the same", and "iso-" means "equal".

Something homogeneous is "the same throughout" rather than "equal throughout". Something isotropic gives "equal measurements in different directions", rather than "the same measurement in different directions".

The difference in meaning is kind of subtle, but it's there.

iso- from Greek isos equal
homo- via Latin from Greek, from homos same
One would therefore expect words of Greek origin to take the iso- prefix and those of Latin origin take the homo- prefix. (Does that hypothesis hold?)

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    Hmm, I don't think it holds, homologue, homophobe, homogenous, homeostasis, homeomorph, homeopath, homonym and homozygous all come from Greek roots. Since both homo- and iso- are such common prefixes, I think you'll find that both of them are quite promiscuous as used in English and will go with just about anything.
    – terdon
    Commented Oct 14, 2013 at 13:41
  • @Kris: as you said homo- has greek origin too. Words like homophony, homology, homotopy and homophobia all contain greek substantives, so apparently (and unfortunately) etymology doesn't give us an easy way out by having latin words attached to homo-
    – Matthaeus
    Commented Apr 24, 2014 at 13:06
  • @terdon homeo- (or homoeo-) is a different prefix, meaning "similar", though. Just to keep things confusing.
    – Vaz
    Commented Jan 25, 2019 at 2:55
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Milk is "homogenized", meaning that it's been mixed and shaken and stirred until every part is like every other part: "all the same kind of stuff".

An "isotope" of an element is another form with the same number of protons (that's the "equal" (equal number)) part, but different numbers of neutrons.

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