letter (written character) / letter (written message) / the literal meaning, the plain meaning of something

suicide note (message, usually a written note, left by someone who later attempts or commits suicide, usually explaining the reason for the suicide)

love letter

open letter

letter of intent (legal document outlining one or more agreements between two or more parties before the actual agreements are finalized)

newsletter

to be ignored, not to be taken into account, to go unheeded, to have no effect

bread-and-butter letter, bread-and-butter note

letters patent of nobility / good reputation

letter of recommendation