letter, missive -- (a written message addressed to a person or organization; "mailed an indignant letter to the editor")
letter, letter of the alphabet, alphabetic character -- (the conventional characters of the alphabet used to represent speech; "his grandmother taught him his letters")
letter -- (owner who lets another person use something (housing usually) for hire)
letter -- (a strictly literal interpretation (as distinct from the intention); "he followed instructions to the letter"; "he obeyed the letter of the law")
letter, varsity letter -- (an award earned by participation in a school sport; "he won letters in three sports")