The small box on the far-left side in the title bar of an active window. Clicking a close box closes the window.

A plain box on the left end of the title bar. Clicking this will cause the window to close.

interface: A small square in the upper left corner of a window which, when clicked, closes the window. Inside an application, closing the window removes one view of a document, and is not the same as Quitting, though some applications will inappropriately quit upon this action.