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The Collaborative International Dictionary of English v.0.48 (gcide)

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]