Part of Speech: noun neuter
Neuter of a presumed derivative of the base of G3702; a relish to other food (as if cooked sauce), that is, (specifically) fish (presumably salted and dried as a condiment): - fish.
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 ὀψάριον; click on the second column to search for that grammatical form of the root ὀψάριον; click on the third column to search for that word and grammatical form; click on the total to search for the root ὀψάριον.