Elision is the expulsion of a short vowel at the end of a word before a word beginning with a vowel. An apostrophe (') marks the place where the vowel is elided.
ἀλλ' (ὰ) ἄγε, ἔδωκ' (α) ἐννέα, ἐφ' (= ἐπὶ) ἑαυτοῦ ( cross64), ἔχοιμ' (ι) ἄν, γένοιτ' (ο) ἄν.
a. Elision is often not expressed to the eye except in poetry. Both inscriptions and the Mss. of prose writers are very inconsistent, but even where the elision is not expressed, it seems to have occurred in speaking; i.e. ὅδε εἶπε and ὅδ' εἶπε were spoken alike. The Mss. are of little value in such cases.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].