What does the root "ject" mean? It occurs in words such as "subject", "object", "project", "injection", "surjection", "bijection". As far as I know these words came to English from French and, in turn, from Latin. Prefixes "ob-", "sub-", "pro-" also came from Latin as Wiktionary says.

1 Answer 1


The root is Latin iaciō (throw, cast), whose supine is iactum.

Because of Latin ablaut (vowel change), prefixes like sub-, ob-, pro- trigger a vowel change to *-iectum.

  • More grammatical terms (many used in mathematics as well) and their Latin derivations are available here.
    – jlawler
    Commented Aug 25, 2014 at 15:36
  • 1
    This isn't ablaut, as the term is usually used, but unstressed vowel weakening.
    – TKR
    Commented Aug 25, 2014 at 17:23
  • I'm not actually sure what the literature calls it -- possibly vowel mutation. But it's not called unstressed vowel weakening -- for one, the changed vowel in proiectum is actually stressed...
    – jogloran
    Commented Aug 26, 2014 at 4:49
  • Naturally, it's not ablaut, no prefix triggers it, it's the very fact of derivation that does, it's just a historical vowel alternation. The rule for this a > e change is: Short ă in a closed non-final and in a closed final syllable of a multi-syllable word changes to short ĕ. E. g.: Princĕps < *prim-căp-s (primus + căpio 'first taking'), here in a closed final syllable ă > ĕ.
    – Yellow Sky
    Commented Aug 26, 2014 at 13:07
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    @jogloran This is about eight and a half years late, but yes, it is unstressed vowel weakening. The weakening happened at a pre-Latin stage when the language had fixed initial stress; so ˈprōiaciō, ˈprōiactum become ˈprōiciō, ˈprōiectum just like ˈprincaps, ˈprincapes/-os (from *ˈprīmo-cap-) becomes ˈprinceps, ˈprincipis. Then later on, stress distribution changed to the Classical paenult-or-antepaenult position, but by then the vowels had already changed. Commented Mar 17, 2023 at 2:07

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