A diptote – pronounced as a dissyllable, with the ‘e’ silent, the stress on the first syllable, which is short, and the second syllable rhyming with ‘boat’ or ‘goat’ – is a noun or adjective which has (only) two case endings.
There is little need for the word ‘diptote’ in the study of English grammar, since nouns and adjectives in English are not declined, i.e., do not take different endings to mark their different functions (e.g., subject or object) in a sentence. However, the word is often used in the study of, e.g., Ancient Greek or Arabic. Thus in Modern Standard Arabic most nouns have three case endings – for the nominative, accusative, and genitive cases – but a number of nouns, because of their structure, have only two case endings, the accusative and genitive sharing the same ending. These nouns are commonly referred to as ‘diptotes’.
Etymological note: ‘Diptote’ and ‘triptote’ come from the Ancient Greek diptotos and triptotos, which are compounds, respectively, of di- (two) and tri- (three) and ptosis (fall, falling, or (in Grammar) the mode or modification of a word). Diptotos and triptotos have the same meaning as their English derivatives.