1. A slot or opening in a door or wall through which mail is delivered.
Usage: Common in British English; American English often uses 'mailbox' or 'mail slot'.
2. A public box or receptacle for depositing outgoing mail.
Usage: Chiefly British English.

1. To display (a film or video) in letterbox format, with black bars at the top and bottom to preserve the original aspect ratio.
Usage: Technical; used in film and television.

Examples

• She checked the letterbox every morning for bills and packages.
• The red letterbox on the corner is where residents post their letters.
• I dropped the invitation through the letterbox of my neighbor's house.
• The classic film was letterboxed on the DVD to maintain its original widescreen format.
• The letterbox format preserves the director's intended composition.
• He walked past three letterboxes on his way to work.