facultative
Concise Oxford English Dictionary © 2008 Oxford University Press:
facultative /ˈfak(ə)lˌtətɪv/
▶adjective
- 1 occurring optionally in response to circumstances rather than by nature.
- 2 Biology capable of but not restricted to a particular function or mode of life: a facultative parasite.
– derivatives
facultatively adverb.
facultatively adverb.
'facultative' also found in these Oxford entries:

