The headword is a rare feminine noun in the nominative singular; see generally LSJ s.v., and cognates at e.g. tau 31, tau 32, tau 33, tau 34, and tau 35. Its appearance here is very probably generated by its appearance in 'Antiphon', noted by Pollux 9.53; that attestation, its earliest, is variously attributed to A. the orator (fr. 179 Blass) and A. the sophist (87 B 42 Diels-Kranz).
[1] Identical glossing in the Synagoge (tau 13) and Photius' Lexicon (tau22 Theodoridis); and compare Hesychius tau61 (s.v. talantw/sei) and Pollux 9.53. The glossing is with two other feminine nouns in the nominative singular. The first of them is transmitted as sth=sis which occurs only here and is not registered by LSJ. If it exists, it is presumably cognate with the verb i(/sthmi, I make to stand (see LSJ s.v. IV.1-2, with the additional senses of I place in the balance, weigh, weigh out) and it might be rendered into English as assessing. However, Theodoridis (above) obelizes it, and expresses cautious support for Charitonides' conjecture sei=sis shaking. On the second, unproblematic glossing noun, see generally LSJ s.v.