 The terms of second-order logic are defined recursively as follows: Constant symbols and individual variables are terms. If \(t_1,\ldots,t_n\) are terms, U is an n-ary function symbol and F is an n-ary function variable, then \(U(t_1,\ldots,t_n)\) and \(F(t_1,\ldots,t_n)\) are terms. Note that terms denote individuals, not relations or properties. Thus X alone is not a term but x is.