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:
adjectival
common
curt
English
genderless
grandiloquent
latinate
letters
mispelled
mondegreen
noun
olde
on-screen
pentasyllabic
polysyllabic
pronounceable
readable
sesquipedalian
shibboleth
short
semordnilap
trochee
unhyphenated
useful
word
wee
/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.