An antepenultimate syllable is the syllable that precedes the penultimate syllable and final syllable in a trisyllabic (or greater) word. In a tetrasyllabic (or greater) word, it is preceded by a preantepenultimate syllable.
English nouns are often stressed on this syllable, with the two subsequent syllables unstressed, as is the syllable that precedes the antepenultimate syllable. For example, capacity /kə'pæsətɪ/ follows a oOoo stress pattern. Other examples include advertisement /əd'vɜ:tɪsmənt/ and kilometre /kɪˈlɒmətə/. The general exception to this is with nouns containing -tion, which are stressed on the syllable before -tion; this may happen to be the antepenultimate syllable.
English adjectives ending -ate are also stressed here, e.g. proportionate, moderate, separate.