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]