用户端（Client），是指与服务器相对应，为客户提供本地服务的程序。一般安装在普通的用户机上，需要与伺服端互相配合运行。互联网发展以后，较常用的用户端包括了如万维网使用的网页浏览器，收寄电子邮件时的电子邮件用户端，以及即时通信的用户端软件等。

A client is a piece of computer hardware or software that accesses a service made available by a server. The server is often (but not always) on another computer system, in which case the client accesses the service by way of a network. The term applies to the role that programs or devices play in the client–server model.