Definitions of letter:

• noun:   a written message addressed to a person or organization

  Example: "Mailed an indignant letter to the editor"

• noun:   an award earned by participation in a school sport

  Example: "He won letters in three sports"

• noun:   the conventional characters of the alphabet used to represent speech

  Example: "His grandmother taught him his letters"

• noun:   a strictly literal interpretation (as distinct from the intention)

  Example: "He followed instructions to the letter"

• noun:   owner who lets another person use something (housing usually) for hire
• verb:   win an athletic letter
• verb:   mark letters on or mark with letters
• verb:   set down or print with letters