Full Idea
If a well-formed formula of propositional calculus takes at least one T and at least one F for all the assignments of truth-values to its variables, it is said to be 'contingent'.
Gist of Idea
A wff is 'contingent' if produces at least one T and at least one F
Source
E.J. Lemmon (Beginning Logic [1965], 2.3)
Book Reference
Lemmon,E.J.: 'Beginning Logic' [Nelson 1979], p.68