Definition of "facultative" [fac•ul•ta•tive]

• Of or relating to a mental faculty. (adjective)
• Capable of occurring or not occurring; contingent. (adjective)
• Not required or compulsory; optional. (adjective)
• Granting permission or authority. (adjective)
• Biology Capable of functioning under varying environmental conditions. Used of certain organisms, such as bacteria that can live with or without oxygen. (adjective)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "The Turkish children featured in the documentary show what I'd call facultative quadrupedalism; they walk on all fours out of necessity."
• "Most of the bacteria which produce disease are facultative, that is, they grow either with or without oxygen; but certain of them, as the bacillus of tetanus, are anaërobic."
• "In some cantons the referendum is obligatory, in others it is "facultative," or optional."