substitution instance

noun1

1. •
   A well-formed formula (wff) that has a constant term in place of a variable; expressed as Q(a/x), where the resulting wff is identical to Q except that the constant a takes the place of every free occurrence of the variable x.

   • The result of substituting a term a for all free occurrences of the variable x in the formula Q is denoted by Q(a/x). — 1972