Meaning of “letter” - Learner’s Dictionary

letter

noun [ C ] us uk /ˈletər/
MESSAGE

A1 a written message that you send to someone, usually by post:

I got a letter from Paul this morning.

More examples

SYMBOL

A2 a symbol that is used in written language and that represents a sound in that language:

the letter K
→ See also covering letter

(Definition of “letter” from the Cambridge Learner’s Dictionary © Cambridge University Press)