Apollyon = "Destroyer"
1) the angel of the bottomless pit, the Destroyer
Part of Speech: noun proper masculine
Citing in TDNT: 1:397, 67
Active participle of G622; a destroyer (that is, Satan): - Apollyon.
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 Ἀπολλύων; click on the second column to search for that grammatical form of the root Ἀπολλύων; click on the third column to search for that word and grammatical form; click on the total to search for the root Ἀπολλύων.