ex is a powerful line based text editor (i.e. you don't see the contents of the file while editting) for UNIX(-y) systems.
Although it is not as popular for stand-alone use as the screen oriented interface (vi), a lot of vi users use ex commands.
The ex editor can be entered from visual mode (vi) by pressing ESC-:. The most popular use of the line oriented interface is replacing large portions of text.
The first sighting of ex appears to be in AT&T UNIX version 7, which was released in 1979.