The Free On-line Dictionary of Computing (30 December 2018):

client-server
client/server
client-server model

    A common form of distributed system in which
   software is split between server tasks and client tasks.
   A client sends requests to a server, according to some
   protocol, asking for information or action, and the server
   responds.

   This is analogous to a customer (client) who sends an order
   (request) on an order form to a supplier (server) who
   despatches the goods and an invoice (response).  The order
   form and invoice are part of the "protocol" used to
   communicate in this case.

   There may be either one centralised server or several
   distributed ones.  This model allows clients and servers to be
   placed independently on nodes in a network, possibly on
   different hardware and operating systems appropriate to
   their function, e.g. fast server/cheap client.

   Examples are the name-server/name-resolver relationship in
   DNS, the file-server/file-client relationship in NFS and
   the screen server/client application split in the X Window
   System.

   Usenet newsgroup: news:comp.client-server.

   ["The Essential Client/Server Survival Guide", 2nd edition,
   1996].

   (1998-01-25)