fac·ul·ta·tive 
(făkəl-tā′tĭv)
(făkəl-tā′tĭv)adj.
1. Of or relating to a mental faculty.
2.
a. Capable of occurring or not occurring; contingent.
b. Not required or compulsory; optional.
3. Granting permission or authority.
4. Biology Capable of functioning under varying environmental conditions. Used of certain organisms, such as bacteria that can live with or without oxygen.
facul·ta′tive·ly adv.