Full Idea
A 'substitution-instance' is a wff which results by replacing one or more variables throughout with the same wffs (the same wff replacing each variable).
Gist of Idea
A 'substitution-instance' is a wff formed by consistent replacing variables with wffs
Source
E.J. Lemmon (Beginning Logic [1965], 2.2)
Book Reference
Lemmon,E.J.: 'Beginning Logic' [Nelson 1979], p.53