(eXit button) Also called a "close" or "exit" button, clicking or tapping the X removes the current window, dialog box or popup message from the screen. It is also used to delete text and graphics.
Exit Button - Windows Vs. Mac
The X in Windows is always visible and clicking it closes the application. The X in the Mac appears only when the cursor is above it. The X also closes the app, but the Mac keeps it in memory for a quick relaunch. Of course, the X's are on opposite corners of the window. See Mac Dock.
Clear the Line
In smartphones and tablets, the X button clears the text in the search box. The X on the keyboard is the backspace key.