That which asks, in a system where something else answers. Also one served under an obligation of care.
Latin cliens/clientem (a follower, a dependent), from clinare (to lean, to bend); a client was, at the root, one who leaned on a patron for protection and favour.
Both senses put the asking party in the weaker position — a client depends on an answer it cannot compel, whether the one owing that answer is a server across a network or a professional bound by duty, and the obligation runs in one direction only.