A word, sentence, paragraph or story that describes itself - like this one.
Only the fool would take trouble to verify that his sentence was composed of ten a's, three b's, four c's, four d's, forty-six e's, sixteen f's, four g's, thirteen h's, fifteen i's, two k's, nine l's, four m's, twenty-five n's, twenty-four o's, five p's, sixteen r's, forty-one s's, thirty-seven t's, ten u's, eight v's, eight w's, four x's, eleven y's, twenty-seven commas, twenty-three apostrophes, seven hyphens and, last but not least, a single! -- LeeSallows?
Also known as reflexive.
Also known as "homological" - as opposed to "heterological", which of course is a word, etc., that does not describe itself. Needless to say, asking oneself whether the word "heterological" is heterological is unwise.
"This song's just six words long" -- Weird Al Yankovic
http://www.lyricsdomain.com/lyrics/3233/
Similar: "Title of the Song" -- Da Vinci's Notebook
http://www.bobandtom.com/gen3/title_song_windoid.html