1) to look at, behold
2) to allow one's self to be seen, to appear
Part of Speech: verb
Citing in TDNT: 5:315, 706
The first a (middle voice) prolonged form of the second (primary) which is used for it in certain tenses; and both as alternates of G3708; to gaze (that is, with wide open eyes, as at something remarkable; and thus differing from G991, which denotes simply voluntary observation; and from G1492, which expresses merely mechanical, passive or casual vision; while G2300, and still more emphatically its intensive G2334, signifies an earnest but more continued inspection; and G4648 a watching from a distance): - appear, look, see, shew self.
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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 ὀπτάνομαι; click on the second column to search for that grammatical form of the root ὀπτάνομαι; click on the third column to search for that word and grammatical form; click on the total to search for the root ὀπτάνομαι.