Found:

propositional letters

**SEMANTICS: **the basic expressions of a propositional language (also
called propositional variables, as opposed to
**logical constant**s like Neg and &).
Conventionally, a letter from the range {p, q, r or a p with primes (p',p'',...) represents a single proposition. The propositional formulas of propositional logic
are built from propositional letters and connectives. Propositional letters should be distinguished from metavariables such as phi and psi which are used in the
definitions of the logic. See also
**propositional logic**.

LIT. | Gamut, L.T.F. (1991) |