Optional, 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.
Shades of meaning
Fernald lists optional only as an antonym, under necessary.
He lists it without comment; the entries above explain its neighbors.
Synonyms
none given