The headword -- already under nu 429 -- is a neuter substantive (tmh=ma, -atos, to/) and given here in the nominative (and vocative and accusative) singular; cf. kappa 149 (gloss) and LSJ s.v. In mathematical contexts the headword means segment (Netz, p. 24), either of a straight line or of a circular arc (Euclid, Elements III, Definitions 6, 7, 8, 11; cf. Heath, pp. 1-5).
[1] Like the lemma, the gloss is a neuter noun in the nominative, vocative, and accusative singular; see LSJ s.v. One of its meanings is identical. However, it also signifies the minting or impression of a coin; cf. kappa 262 (end), kappa 1981, and nu 466 (end).