Client
The term "client" indicates a component that accesses to the (remote) services or resources of another component, named server. So in this contest the client referees to software or hardware. A computer connected to a server through some kind of a network that demands for one or more services, using the network protocols, is an example of a hardware client. An e-mail program is an example of a software client. The term "client" also indicates a software used on a computer to accede to the functionalities offered by a server.
For example, on the web the software client is a browser, it connects with a web server through the HTTP protocol, for an e-mail a client is a mail user agent (Outlook, Mozilla Thunderbird, Eudora, it connects to a server due to SMTP and POP or IMAP protocols, a client for a consultation or database modification connects to a DBMS, that manages the database e answers to client's demands.
There are more and more software, as web, e-mail, database, that are divided to a client-part (resident and executed on the pc client) and a server-part (resident and executed on the server).