WordNet
n. a push button on the mouse
Wikipedia
A mouse button is a microswitch on a computer mouse which can be pressed (“clicked”) to select or interact with an element of a graphical user interface.
The three-button scrollmouse has become the most commonly available design. As of 2007 (and roughly since the late 1990s), users most commonly employ the second button to invoke a contextual menu in the computer's software user interface, which contains options specifically tailored to the interface element over which the pointer currently sits. By default, the primary mouse button sits located on the left-hand side of the mouse, for the benefit of right-handed users; left-handed users can usually reverse this configuration via software.
Usage examples of "mouse button".
Finally they figured out that these network crashes were triggered whenever a user, scanning the menus for a particular item, held down the mouse button for more than a couple of seconds.
He chanced a look at Gina, as he clicked the mouse button to connect again with UNA3, and then clicked ‘.