The copulative a (also a copulativum, a athroistikon) is the prefix ἁ- (ha-) or α- (a-) used to express unity in Ancient Greek, derived from Proto-Indo-European *sm̥-, cognate to English same (see also symbel).
[1] An example is ἀδελφός (adelphós 'brother'), from *sm̥-gʷelbhos, literally meaning 'from the same womb' (compare Delphi).
In Proto-Greek, the Proto-Indo-European phoneme *s at the beginning of a word became *h by debuccalization and syllabic *m̥ became *a, giving the combined form *ha-.
Less exact cognates include English same and some, and Latin simul 'at the same time' and similis 'similar'.
[2][3] Other words in Greek are related, including ᾰ̔́μᾰ (háma 'at the same time'), ὁμός (homós 'same'), and εἷς (heís 'one'; from Proto-Indo-European *sem-s).