Instantiation

A application of a Substitution to all of the free variables in a Formula or Term.

if $S$ is a substititon, $T$ is a formula or term, $ST$ is the instantiation

• $T=\text{parent}(X,Y)$
• $S=\{X/sue,Y/bob\}$
• $ST=\text{parent}(sue,bob)$