mouse-highlight

mouse-highlight is a variable defined in `C source code'.
Its value is

t

Documentation:
If non-nil, clickable text is highlighted when mouse is over it.
If the value is an integer, highlighting is only shown after moving the
mouse, while keyboard input turns off the highlight even when the mouse
is over the clickable text. However, the mouse shape still indicates
when the mouse is over clickable text.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.