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An autological word (also called homological word) is a word expressing a property which it also possesses itself (e.g.., the word "short" is short, "noun" is a noun, "English" is English, "pentasyllabic" has five syllables, "word" is a word, "sesquipedalian" is a long word, see  for a partial list). The opposite is a heterological word, one that does not apply to itself (e.g., "long" is not long, "verb" is not a verb, "phonetic" is not spelled the way it sounds, "monosyllabic" has five syllables, "German" is not German, etc.).
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.
The fame of this paradox later extending also to non-academic circles has created a more widespread popular interest, expressing itself in more recent times also in the creation of lists of autological words.
One source of autological words are archetypal words (ostensive definition) – words chosen to describe a phenomenon by using an example of the phenomenon, which are thus necessarily autological. One such example is a mondegreen – a mishearing of a phrase, which itself is based on a mishearing of "And laid him on the green" as "And Lady Mondegreen".
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.
The word autological itself may or may not be an autological word. It demonstrates an infinite regress: any word is autological if its appearance expresses its own meaning, so autological is autological if autological expresses the property of a word expressing its own meaning. Whether the word 'heterological' is itself heterological is an even more problematic paradox.
- 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")
- 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)
- Henry Segerman: Autological words; Wiktionary: English autological terms