Major General Panic's definition below still doesn't seem to match any Lisp (or Scheme) I've ever seen in use, in various subtle ways, so I prefer to keep this writeup. While it is shorter -- a fact that earns it downvotes -- I believe it to be correct.
Drawing on the Greek tradition, in Lisp, Prolog, most functional programming languages (and Scheme and Emacs Lisp and YAPL...), an atom is any ``indivisible'' entity, such as a symbol, number, character, ...
Contrast various "containers", such as lists, arrays and structures, which hold subobjects — they're divisible.
The question of whether NIL is atomic or not is great fun for the budding undergraduate philosopher/troll set.