Definitions for "X client"
an applications program that communicates with the server, sending it requests such as "draw a line" or "pay attention to keyboard input
an application that draws graphics to a screen in the X environment, and gets input from the devices attached to that screen
an application that wants to display a window on the server, like a web browser or email application
a program that opens and uses windows, such as a browser, email client, word-processor and so on