To press the middle button on a computer mouse.(verb)
To activate an item on the screen by middle-clicking while the pointer is over it.(verb)
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Use "middle-click" in a sentence
"You can configure a corner of the touchpad to act as a middle-click button."
"Say, middle-click that button to maximize vertically and right-click it to maximize horizontally."
"For more, check out how Windows 7 creates new folders with a hotkey, maximises windows vertically with a double-click, and closes applications with a middle-click."