An autological word is, simply, a word that describes itself. It is deliciously fun both to think up these words and to decipher them. Some examples of autological words in English include:
/msg me if you think of more!
The opposite of an autological word would be a "heterological" word, which would describe the vast majority of words in any language.
The question of whether the words "autological" and "heterological" are themselves autological or heterological words is the subject of the Grelling–Nelson Paradox, a semantic paradox formulated in 1908 by philosophers Kurt Grelling and Leonard Nelson.