:N-ary feature:

Pronunciation ˈenəri

A phonological feature, which unlike binary features and elements, can take more than two values. In The sound pattern of English stress is treated in this way, vowels being capable of being marked as [1 stress], [2 stress], [3 stress], and so on.

Chomsky N. & Halle M. (1968) The sound pattern of English. New York: Harper & Row.