The headword, a neuter noun in the nominative/accusative singular, is attested in this form only in lexicography. Euripides uses it numerous times in plural oblique forms, e.g. Alcestis 873 (web address 1).
cf. alphaiota 4 for a related verb.
[1] Likewise, according to Adler, in the Ambrosian Lexicon. The corresponding entry in ps.-Zonaras (s.v.), which is clearly derived from the same source, applies this gloss differentially to a word similar (and, in late Greek, homophonous) to the headword, e)/agma.