To press the left button on a computer mouse.(verb)
To activate an item on the screen by left-clicking while the pointer is over it.(verb)
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Use "left-click" in a sentence
"Surprise, the left-click on my trackball was all worn out!"
"He also changed the left-click behaviour to switch to the last window you were using of a particular application instead of popping up the preview list — clicking more than once will cycle through the list of open windows, and hovering over the button will still show the thumbnail previews."
"Highlight (left-click and drag) to see the hidden text."