functor, functor
In Prolog, the word functor is used to refer to the atom at the start of a structure, along with its arity, that is, the number of arguments it takes. For example, in likes(mary, pizza), likes/2 is the functor. In a more complex structure, like

persondata(name(smith, john), date(28, feb, 1963))

the top-level functor is termed the principal functor - in this case persondata/2 - There is also a built-in predicate called functor, used to extract the name part and arity of a structure.

This built-in predicate takes three arguments: functor(Term, Name, Arity). It succeeds if Term is a term with functor name Name and arity Arity. Examples:

?- functor(likes(mary, pizza), Name, Arity).
Name = likes
Arity = 2 

?- functor(likes(X, Y), Name, Arity).
X = _G180
Y = _G181
Name = likes
Arity = 2

?- functor(likes, Name, Arity).
Name = likes
Arity = 0 

?- functor(X, likes, 2).
X = likes(_G232, _G233) 

Sometimes there are reasons to want to have the functor name somewhere other than at the start of the structure. For example, in the expression X < Y, "</2" is the functor, and so "<" is the functor name:

?- functor(2 < 4, Name, Arity).
Name = (<),
Arity = 2.

?- 2 < 4.
true.

?- <(2, 4).
true.
See op to find out how this works.

The term functor is used in a different sense in mathematics and in functional programming, and a different way again in philosophy.