1) to put one's attention on, fix one's thoughts on, to turn one's self or one's mind to, put one's thought upon a thing
1a) to be acquainted with, to understand
1b) to know
Part of Speech: verb
Apparently a middle voice of G2186 (with G3563 implied); to put the mind upon, that is, comprehend, or be acquainted with: - know, understand.
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 ἐπίσταμαι; click on the second column to search for that grammatical form of the root ἐπίσταμαι; click on the third column to search for that word and grammatical form; click on the total to search for the root ἐπίσταμαι.