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 German).
Unlike more general concepts of autology and self-reference, this particular distinction and opposition of "autological" and "heterological words" is uncommon in linguistics for describing linguistic phenomena or classes of words, but is current in logic and philosophy where it was introduced by Kurt Grelling and Leonard Nelson for describing a semantic paradox, later known as Grelling's paradox or the Grelling–Nelson paradox.
One source of autological words is ostensive definition: the reference to a class of words by an example of the member of the class, as it were by synecdoche: such as mondegreen, oxymoron, eggcorn, bahuvrihi, etc. A word's status as autological may change over time. For example, neologism was once an autological word but no longer is; similarly, protologism (a word invented recently by literary theorist Mikhail Epstein) may or may not lose its autological status depending on whether or not it gains wider usage.
^Grelling and Nelson used the following definition when first publishing their paradox in 1908: "Let φ(M) be the word that denotes the concept defining M. This word is either an element of M or not. In the first case we will call it 'autological', in the second 'heterological'." (Peckhaus 1995, p. 269). An earlier version of Grelling's paradox had been presented by Nelson in a letter to Gerhard Hessenberg on 28 May 1907, where "heterological" is not yet used and "autological words" are defined as "words that fall under the concepts denoted by them" (Peckhaus 1995, p. 277)
Volker Peckhaus: The Genesis of Grelling's Paradox, in: Ingolf Max / Werner Stelzner (eds.), Logik und Mathematik: Frege-Kolloquium Jena 1993, Walter de Gruyter, Berlin 1995 (Perspektiven der analytischen Philosophie, 5), pp. 269–280
Simon Blackburn: The Oxford Dictionary of Philosophy, Oxford University Press, 2nd ed. Oxford 2005, p. 30 ("autological"), p. 170 ("heterological"), p. 156 ("Grelling's paradox")