Definition:Logical Formula

Definition
A logical formula, or propositional formula (or, when the context is understood, just formula) is a particular example of a statement form.

It is traditional, particularly in the field of mathematical logic, to use lowercase Greek letters to stand for general formulas (the usual ones being $\phi, \psi$ and $\chi$), but more modern treatments are starting to use ordinary lowercase letters of the English alphabet, usually $p, q, r$ etc.

Linguistic Note
The plural of formula is usually given nowadays as formulas, but the Classical plural formulae (or even formulæ) is frequently seen, particularly in older literature.