Definition:Formal Language/Alphabet/Letter



A letter of a formal language is a more or less arbitrary symbol whose interpretation 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.

Bourbaki Definition
The mathematical theory as defined in 's specifies the letters as follows:

The letters of a mathematical theory are uppercase and lowercase Roman letters, with or without accents, e.g. $A, A', A''$.

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