Definition: optional

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Source: WordNet (r) 1.7 

optional
     adj : possible but not necessary; left to personal choice [ant: obligatory]

Source: Webster's Revised Unabridged Dictionary (1913) 

Optional \Op"tion*al\, a.
   Involving an option; depending on the exercise of an option;
   left to one's discretion or choice; not compulsory; as,
   optional studies; it is optional with you to go or stay. --
   n. See Elective, n.

         If to the former the movement was not optional, it was
         the same that the latter chose when it was optional.
                                                  --Palfrey.

         Original writs are either optional or peremptory.
                                                  --Blackstone.