optional

[op-shuh-nl]
adjective
1.
left to one's choice; not required or mandatory: Formal dress is optional.
2.
leaving something to choice.

Origin:
1755–65; option + -al1

optionality, noun
optionally, adverb
nonoptional, adjective
nonoptionally, adverb
unoptional, adjective
unoptionally, adverb


1. discretional, elective, voluntary.
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
optional (ˈɒpʃənəl)
 
adj
possible but not compulsory; left to personal choice
 
'optionally
 
adv

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Etymonline
Word Origin & History

optional
in reference to things which may be done or not done, 1792, from option.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
When nominations are made it is optional whether they are seconded or not.
We may give others a preference over him in optional good offices, except those
  which tend to his improvement.
Shellfish are optional, but always add glamour and color if you wish to include
  them.
Frying the crab first is optional, but it adds superb flavor and makes the
  crabmeat more tender.
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