mouse-position

mouse-position is a built-in function in `C source code'.

(mouse-position)

Return a list (FRAME X . Y) giving the current mouse frame and position.
The position is given in canonical character cells, where (0, 0) is the
upper-left corner of the frame, X is the horizontal offset, and Y is the
vertical offset, measured in units of the frame's default character size.
If Emacs is running on a mouseless terminal or hasn't been programmed
to read the mouse position, it returns the selected frame for FRAME
and nil for X and Y.
If `mouse-position-function' is non-nil, `mouse-position' calls it,
passing the normal return value to that function as an argument,
and returns whatever that function returns.