letter. 1. A written communication that is usu. enclosed in an envelope, sealed, stamped, and  delivered (esp., an official written communication) <an opinion letter>.2. (usu. pl.) A written instrument containing or affirming a grant of some power or right <letters testamentary>. [Cases: Executors and Administrators  27. C.J.S. Executors and Administrators § 79.] 3. Strict or literal meaning <the letter of the law>. • This sense is based on the sense of a letter of the alphabet. Cf. SPIRIT OF THE LAW.

[Blacks Law 8th]