1) a helmet
2a) the protection of the soul which consists in (the hope of) salvation
Part of Speech: noun feminine
Citing in TDNT: 5:314, 702
Feminine of a compound of G4012 and G2776; encirclement of the head, that is, a helmet: - helmet.
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 περικεφαλαία; click on the second column to search for that grammatical form of the root περικεφαλαία; click on the third column to search for that word and grammatical form; click on the total to search for the root περικεφαλαία.