1. A letter which lies for a certain period uncalled-for at a post office, and is then sent to the General Post Office to be opened; a written law that has ceased to be kept.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• sead letter,
• xead letter,
• cead letter,
• fead letter,
• read letter,
• eead letter,
• dwad letter,
• dsad letter,
• ddad letter,
• drad letter,
• d4ad letter,
• d3ad letter,
• dezd letter,
• desd letter,
• dewd letter,
• deqd letter,
• deas letter,
• deax letter,
• deac letter,
• deaf letter

The adjective form of the term "dead letter" is "dead-letter".

The plural form of "dead letter" is "dead letters".

The singular form of the word "dead letter" is "dead letter". The word does not change when used in singular or plural form.