atom
An atom, in Prolog, means a single data item. It may be of one of four types:

Numbers, in Prolog, are not considered to be atoms.

atom is also the name of a built-in predicate that tests whether its single argument is an atom.

?- atom(pizza).
true.

?- atom(likes(mary, pizza)).
false.

?- atom(<-->).
true.

?- atom(235).
false.

?- X = some_atom, atom(X).
X = some_atom.
The final example means that atom(X) has succeeded, with X bound to some_atom.