facultative

adj

1. Of or relating to faculty, especially to mental faculty.
2. 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.”

   “The policy would be a “facultative” policy, whereby the insurers would first have assessed the risk involved in the particular shipment and decided to accept it.”

3. That grants permission or power to do something.
4. Able to perform a particular life function, or to live generally, in more than one way.

   “facultative feeder”

   “This hypothesis might be supported by the evidence that there are varieties of pathogenic fungi of which parasitism is in varying degrees, between obligate parasitic and facultative parasitic. The facultative parasite, which usually lives as a saprophyte but under some conditions can parasitize on a plant, seems to be a pathogen with the lowest parasitic adaptation.”

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.”