Achor

trouble, a valley near Jericho, so called in consequence of the trouble which the sin of Achan caused Israel (Josh 7:24; 7:26). The expression "valley of Achor" probably became proverbial for that which caused trouble, and when Isaiah (Isa 65:10) refers to it he uses it in this sense: "The valley of Achor, a place for herds to lie down in;" i.e., that which had been a source of calamity would become a source of blessing. Hosea also (Hos 2:15) uses the expression in the same sense: "The valley of Achor for a door of hope;" i.e., trouble would be turned into joy, despair into hope. This valley has been identified with the Wady Kelt.

EBD - Easton's Bible Dictionary