Lexical Definition

‎πεντηκοστή‎ pentēkostḗ (pen-tay-kos-tay') Pentecost ^G:N-F
feminine of the ordinal of ‎πεντήκοντα‎

Pentecost.

• fiftieth (‎ἡμέρα‎ being implied) from Passover, i.e. the festival of "Pentecost"

Abbott-Smith

πεντηκοστός, -ή, -όν, [in LXX for חֲמִשִּׁים, Le 25:10, 11, IV Ki 15:23, 27; I Mac4, II Mac 14:4; ἡ π., To 2:1, II Mac 12:32*;]
fiftieth. As subst., ἡ π.;
(a) (sc. μερίς), at Athens, a tax of two per cent.;
(b) (sc. ἡμέρα, i.e. the fiftieth day after the Passover), Pentecost, the second of the three great Jewish feasts (To 2:1, II Mac 12:32; ἑορτὴ ἑβδομάδων, De 16:10, al.): Ac 2:1 20:16, I Co 16:8.†

Abbott-Smith References

Liddell-Scott-Jones

Greek of: שָׁבוּעַ

Included with: πεντηκοστή-ός, ή, όν,

fiftieth, [Refs 5th c.BC+]

__II substantive πεντηκοστήή, ἡ,
__II.1 (i.e. μερίς), fiftieth part: hence, tax of two per century. on exports and imports, at Athens, [πεντηκοστήὰ τῶν λίθων “IG” 42(1).103.46 (Epid.); at Delos, [prev. work]22.1635.38 (4th c.BC), 11(2).161 A 26 (3rd c.BC) ">Refs 5th c.BC+]; at Halicarnassus, [Refs 3rd c.BC+]; τῶν πρηθέντων τελείτω π. [Refs 4th c.BC+]; = Latin quinquagesima, [πεντηκοστήήν">Refs 2nd c.AD+] invented a new sort of composition of two per century. in lieu of his cavalry service, i. e. paid this instead of it, [Refs 4th c.BC+]
__II.2 (i.e. ἡμέρα), fiftieth day (after the Passover), [LXX]; ἡ ἡμέρα τῆς Π. [NT]

_{LSJ Scripture Refs: Acts.2.1}