1) that cannot be cut in two, or divided, indivisible
1a) of a moment of time
Part of Speech: adjective
From G1 (as a negative particle) and the base of G5114; uncut, that is, (by implication) indivisible (an "atom" of time): - moment.
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 ἄτομος; click on the second column to search for that grammatical form of the root ἄτομος; click on the third column to search for that word and grammatical form; click on the total to search for the root ἄτομος.