region-active-p

region-active-p is a compiled Lisp function in `simple.el'.

(region-active-p)

Return t if Transient Mark mode is enabled and the mark is active.

Some commands act specially on the region when Transient Mark
mode is enabled. Usually, such commands should use
`use-region-p' instead of this function, because `use-region-p'
also checks the value of `use-empty-active-region'.