say it: ahk-tuh-ji-NER-ee-uhn
octogenarian is a noun and adjective meaning "someone whose age is in the eighties". It carries 2 recorded senses.
someone whose age is in the eighties
being from 80 to 89 years old
Forms marked · are formed by the regular rule rather than recorded in the source data. English keeps a few hundred irregulars and the rest follow the pattern, so the rule is right far more often than not — but it is a rule, and the mark says so.
Definitions, synonyms and semantic relations for octogenarian come from Princeton University’s WordNet. The pronunciation comes from the CMU Pronouncing Dictionary. Commonness is measured against a corpus of everyday English. Full sources and licences.