mouse-set-region

mouse-set-region is an interactive compiled Lisp function in `mouse.el'.

It is bound to <drag-mouse-1>.

(mouse-set-region CLICK)

Set the region to the text dragged over, and copy to kill ring.
This should be bound to a mouse drag event.
See the `mouse-drag-copy-region' variable to control whether this
command alters the kill ring or not.