Terms: Terms are defined inductively by the following clauses.
(i) Every individual variable and every individual constant is a term. (Such a term is called atomic).
(ii) If f is an n-ary function symbol and t1, …, tn are terms then f(t1, …, tn) is a term.
The inductive definition means that terms are exactly all those expressions that can be obtained by a finite number of applications of the clauses. An expression is a term iff this follows from (i) and (ii).