Lexical Definition

‎διαθήκη‎ diathḗkē (dee-ath-ay'-kay) covenant ^G:N-F
from ‎διατίθεμαι‎

covenant, testament.

• properly, a disposition, i.e. (specially) a contract (especially a devisory will)

Abbott-Smith

διαθήκη, -ης, ἡ (< διατίθημι), [freq. in LXX, and nearly always for בְּרִית ;]
1. as usually in cl., a disposition, testament, will (Plat., al.): Ga 3:15 (R, mg., but v. Lft., in l.), He 9:16, 17 (R, txt.; MM, Exp., xi,; Milligan, NTD, 75; Abbott, Essays, 107; Deiss., LAE, 341; but v. infr.).
2. As in LXX (for בְּרִית) = cl. συνθήκη, a convention, arrangement, covenant (exc. in the disputed cases mentioned above, always between God and man, "perhaps with the feeling that the δια- compound was more suitable than the συν- for a covenant with God---συνθ. might suggest equal terms," MM, Exp., l.c.): Ga 3:15 (R, txt., but v. supr., and cf. Thayer, s.v.), He 9:16, 17 (R, mg., Westc., in l.,; Hatch, Essays, 47; but v. supr.), Mt 26:28, Mk 14:24, Lk 1:72, Ac 3:25 7:8, Ro 11:27 (LXX), II Co 3:14, Ga 3:17, He 7:22 8:6, ib. 9, 10 (LXX) 9:4, 15-17 ib. 20 (LXX) 10:16 (LXX), 29 12:24 13:20, Re 11:19; καινὴ δ., Mt 26:28, and Mk 14:24 (R, mg.), Lk 22:20, I Co 11:25, II Co 3:6, He 8:8 (LXX) 9:15; pl. Ro 9:4, Ga 4:24, Eph 2:12.†

Abbott-Smith References

Liddell-Scott-Jones

διαθήκη, ἡ,

(διατίθημι) disposition of property by will, testament, [Refs 5th c.BC+]; κατὰ διαθήκην by will, [Refs 1st c.BC+], etc.: in plural, διαθήκας διαθέσθαι [Refs 5th c.BC+]

__II αἱ ἀπόρρητοι δ. mystic deposits on which the common weal depended, probably oracles (compare διαθέτης), [Refs 4th c.BC+]
__II.2 name of an eyesalve, because the recipe was deposited in a temple, [Refs 6th c.AD+]
__III compact, covenant, ἢν μὴ διαθῶνται διαθήκην ἐμοί [LXX+5th c.BC+]; καινή, παλαιὰ δ., [NT]; disposition (with allusion to [NT]
__IV ={διάθεσις}[Refs 3rd c.AD+]

_{LSJ Scripture Refs: LXX.Gen.6.18 ☩ Luke.22.20 ☩ 2Cor.3.14 ☩ Gal.3.15 ☩ Heb.9.15}