non·a·ge·nar·i·an (nŏn'ə-jə-nâr'ē-ən, nō'nə-) n. A person 90 years old or between 90 and 100 years old.
[From Latin nōnāgēnārius, from nōnāgēnī, ninety each, from nōnāgintā, ninety : nōnus, ninth; see nona- + -gintā, ten times; see dek in Indo-European roots.] non'a·ge·nar'i·an adj.
1804, coined in Eng. from L. nonagenarius "containing ninety" (in L.L. "someone ninety years old"), from nonagen "ninety each," from nonus "ninth" (see nones).