Full Idea
A 'well-formed formula' of the propositional calculus is a sequence of symbols which follows the rules for variables, ¬, →, ∧, ∨, and ↔.
Gist of Idea
A 'well-formed formula' follows the rules for variables, ¬, →, ∧, ∨, and ↔
Source
E.J. Lemmon (Beginning Logic [1965], 2.1)
Book Reference
Lemmon,E.J.: 'Beginning Logic' [Nelson 1979], p.44