Nouns

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

Verbs

(v)letterwin an athletic letter
(v)letterset down or print with letters
(v)lettermark letters on or mark with letters