Anything that is the third from the end; especially, a syllable in a word: The antepenultimate stressed unit in the word immortality is tal, as in im-mor-tal-i-ty.

Two before the last: The antepenultimate comes immediately before the penultimate in a series; for example, a book that has 20 chapters, chapter 18 is the antepenultimate chapter in the book.

Other adjectives indicating such sections in words: ultimate (last), penultimate (one from the last), preantepenultimate (third from the last), propreantepenultimate (fourth from the last).