Lexical Definition

‎τεκμήριον‎ tekmḗrion (tek-may'-ree-on) clear proof ^G:N-N
neuter of a presumed derivative of (a goal or fixed limit)

infallible proof.

• a token (as defining a fact), i.e. criterion of certainty

Abbott-Smith

** τεκμήριον, -ου, τό (< τέκμαρ, a mark, sign), [in LXX: Wi 5:4 19:13, III Mac 3:24 * ;]
a sure sign, a positive proof: Ac 1:3 (for exx., v. MM, xxiv).†
SYN.: ἔνδειγμα, q.v

Abbott-Smith References

Liddell-Scott-Jones

τεκμήριον-ον, τό,

(τεκμαίρομαι) ={τέκμαρ}[Refs 4th c.BC+], a sure sign or token, [Refs 5th c.BC+]; καὶ μὴν στίβοι γε, δεύτερον τ. [Refs]; θανόντος πίστ᾽ ἔχων τ. [Refs 5th c.BC+]; ἀσφαλὲς τ. [Refs 5th c.BC+]: in Medicine texts, a sure symptom, [Refs 2nd c.AD+]

__2 simply sign, symbol, τοῦ φιλαποδήμου [Refs 2nd c.AD+]
__II proof (properly of an argumentative kind, opposed to direct evidence, [Refs 5th c.BC+]; opposed to τὰ εἰκότα, [Refs 5th c.BC+]; but οὐκ εἰκότα τ. [Refs]; τ. τινός proof of a thing, [Refs 5th c.BC+]; τ. δὲ τοῦδε τὸν Ὅμηρον λαβέ (i.e. the case of Homer) [Refs 5th c.BC+]; τ. τινὸς δοῦναι, παρασχέσθαι, [Refs 5th c.BC+]; δείξω, ἐπιδείξω, ἀποδεῖξαι,[Refs 5th c.BC+]
__II.2 τεκμήριονον δέ as an independent clause, now the proof of it is this (which follows), take this as a proof, [Refs 5th c.BC+]; more fully, τ. δέ μοι τούτου τόδε· αἱ μὲν γὰρ φαίνονται κτλ. [Refs 5th c.BC+]; τ. δὲ τούτου καὶ τόδε· παρὰ μὲν Κύρου κτλ. [Refs 5th c.BC+]; χρῆσθαι τεκμηρίῳ ὅτι.. (ὅτι introducing the reason, not the fact) [Refs 5th c.BC+]
__II.3 in the Logic of Aristotle, demonstrative proof, opposed to to the fallible σημεῖον and εἰκός, [Refs 5th c.BC+]