Definition:Letter (Formal Systems)

Definition
A letter of a formal language is a more or less arbitrary symbol whose definition depends on the specific context.

In building a formal language, letters are considered to be the undefined terms of said language.

An important part of assigning semantics to a formal language is to provide an interpretation for its letters.

Letters form the vocabulary of the alphabet of a formal language.

Also see
Compare with signs, which also form part of the alphabet.