The language of a propositional calculus is defined in terms of:

• a set of primitive symbols, called atomic formulas, atomic sentences, atoms, placeholders, prime formulas, proposition letters, sentence letters, or variables, and
• a set of operator symbols, called connectives, logical connectives , logical operators, truth-functional connectives, truth-functors, or propositional connectives.

A well-formed formula is any atomic formula, or any formula that can be built up from atomic formulas by means of operator symbols according to the rules of the grammar. The language , then, is defined either as being identical to its set of well-formed formulas, or as containing that set (together with, for instance, its set of connectives and variables).