<hardware> To press and release a button on a mouse or other pointing device. This generates an event, also specifying the screen position, which is processed by the window manager or application program.
On a mouse with more than one button, the unqualified term usually implies pressing the left-most button (with the right index finger), other buttons would be qualified, e.g. "right-click". Multiple clicks in quick succession, e.g. a double-click, often have a different meaning from slow single clicks. Keyboard modifiers may also be used, e.g. "shift-click", meaning to hold down the shift key on the keyboard while clicking the mouse button.
If the mouse moves while the button is pressed then this is a drag.