Definition of facultative
1a
: of or relating to the grant of permission, authority, or privilege
facultative legislation
b
: optional
2
: of or relating to a mental faculty
3a
: taking place under some conditions but not under others
facultative diapause
b
: exhibiting an indicated lifestyle under some environmental conditions but not under others
facultative anaerobes
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Merriam-Webster unabridged
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Time Traveler for facultative
The first known use of facultative was in 1820
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