εἰκοστός · eikostos — LSJ
twentieth, Od. 5.34, etc.; Ep. also ἐεικοστός Il. 24.765.
εἰκοστή, ἡ, a tax of a twentieth, εἰ. τῶν γιγνομένων, τῶν κατὰ θάλασσαν, Th. 6.54, 7.28.
εἰ. ἐλευθερίας or -ιῶν, = Lat. vicesima manumissionum, IG 3.1446, BGU 96.8, etc.