The Collaborative International Dictionary of English v.0.48:

Optional \Op"tion*al\, a.
   Involving an option; depending on the exercise of an option;
   left to one's discretion or choice; allowed but not
   compulsory; as, optional studies; it is optional with you to
   go or stay. -- n. See Elective, n.
   [1913 Webster]

         If to the former the movement was not optional, it was
         the same that the latter chose when it was optional.
                                                  --Palfrey.
   [1913 Webster]

         Original writs are either optional or peremptory.
                                                  --Blackstone.
   [1913 Webster]

WordNet (r) 3.0 (2006):

optional
    adj 1: possible but not necessary; left to personal choice [ant:
           obligatory]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "optional":
   alternative, arbitrary, autonomous, discretional, discretionary,
   disjunctive, elective, free, free will, gratuitous, independent,
   nonmandatory, offered, proffered, self-acting, self-active,
   self-determined, self-determining, spontaneous, unasked,
   unbesought, unbidden, uncalled-for, uncoerced, uncompelled,
   unforced, uninfluenced, uninvited, unpressured, unprompted,
   unrequested, unrequired, unsolicited, unsought, volitional,
   voluntary, volunteer, willful