1) when, since
1a) of time: after
1b) of cause: since, seeing that, because
Part of Speech: conjunction
From G1909 and G1487; there upon, that is, since (of time or cause): - because, else, for that (then, -asmuch as), otherwise, seeing that, since, when.
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 ἐπεί; click on the second column to search for that grammatical form of the root ἐπεί; click on the third column to search for that word and grammatical form; click on the total to search for the root ἐπεί.