|Definition - An adjective that is being used as a noun.|
Example - Make the rich pay.
(Here rich is an adnoun because it stands for rich people.)
Oxford English Dictionary - Its first citation is from 1753:
"Adnoun, Adnomen, or Adname, is used by some grammarians to express what we more usually call an adjective."
(Chambers Cycl. Supp.)