Wiktionary
letter agreement
n. An agreement, specifically a legally binding contract, reached by the party exchange of letters.
Usage examples of "letter agreement".
This was a simple letter agreement between Mickey Magruder and Tim Littenberg, signed by the latter, in which he acknowledged receipt of the sum of $10,000, a no-interest loan with a five-year balloon payment due and payable five months ago January 15, 1986.