Definition:Letter (Mathematical Theory)

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

Depending on the nature of the mathematical theory, and the particular location in whatever assembly in which it occurs, the interpretation of the letter will vary.

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. $A, A', A''$.