1) ability, force, strength, might
Part of Speech: noun feminine
Citing in TDNT: 3:397, 378
From a derivative of ἱς his (force; compare ἔσχον eschon; a form of G2192); forcefulness (literally or figuratively): - ability, might ([-ily]), power, strength.
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 ἰσχύς; click on the second column to search for that grammatical form of the root ἰσχύς; click on the third column to search for that word and grammatical form; click on the total to search for the root ἰσχύς.