facultative

adjective5

1. •
   Not obligate; optional, discretionary or elective.

   • But does the penny fare end here, said Mr. Nixon, at a merely facultative stop? Surely it ends rather at the station. — 1953
2. •
   Able to perform a particular life function, or to live generally, in more than one way.
3. •
   Of or relating to faculty, especially to mental faculty.
4. •
   That grants permission or power to do something.
5. •
   At which a given function is positive.

   • For then it is seen, as before, that it is the points outside the two sheets which are facultative, and not the points between the surface and the touching plane. — 1866