1) that from which something is surely and plainly known
2) on indubitable evidence, a proof
Part of Speech: noun neuter
Neuter of a presumed derivative of τεκμάρ tekmar (a goal or fixed limit); a token (as defining a fact), that is, criterion of certainty: - infallible proof.
These two sites give similar information, with the definition from several dictionaries and statistics on the use of the word.
Perseus Digital Library
University of Chicago's Logion lexicon
Thayer's dictionary plus other information.
From this site's dictionary (in Italian)
Click on the first column to search for that word as a form of the root τεκμήριον; click on the second column to search for that grammatical form of the root τεκμήριον; click on the third column to search for that word and grammatical form; click on the total to search for the root τεκμήριον.