From German autologisch, from Ancient Greek αὐτός (autós, “self”) + λόγος (lógos, “word”).
An autological word (also called homological word or autonym) is a word that expresses a property that it also possesses (e.g. the word “short” is short, “noun” is a noun, “English” is English, “pentasyllabic” has five syllables, “word” is a word). The opposite is a heterological word, one that does not apply to itself (e.g. “long” is not long, “monosyllabic” has five syllables, “German” is not a German word). The world “neologism” was once autological, but now that it is no longer a new word, it is no longer autological.
The opposite of an autological word is a heterological word. Whether or not “heterological” is in fact a heterological word or an autological word is a paradox.