op·tion·al I. \-shənəl, -shnəl\adjective : involving or depending on the exercise of an option : left to the discretion of the one concerned : not compulsory or obligate < an optional activity > < formal dress is optional > • op·tion·al·i·ty\ˌäpshəˈnaləd.ē\noun-es • op·tion·al·ly\ˈäpshənəlē, -shnəlē, -li\adverb II. noun (-s) 1.: something that is optional 2.chiefly Britain: elective