(archaic) The letter r.

(computing) An alphabetical identifier assigned to a drive or volume on MS-DOS or Windows.

reference letter

plural of letter writer

Any of the first seven letters of the alphabet, used in religious calendars to mark the Sundays throughout the year and determine dates such as Easter Sunday.

plural of letter grade

plural of letter opener

A letter written in superscript, usually when it indicates an abbreviation.

(US, law) A written agreement between a prosecutor and a defendant or prospective witness, allowing the latter to give the prosecutor information about an alleged crime, while limiting the prosecutor's ability to use that information against them.

(philately) A sheet of paper that may be folded (and often sealed) and mailed without the use of an envelope.