IPA: /ˈæd.nəʊn/
KK: /ædˈnaʊn/
A word that describes a noun but is used in a sentence as if it were a noun itself. For example, in the phrase 'the rich', 'rich' is an adjective that acts like a noun.
In the phrase 'the merciful', the word 'merciful' acts as an adnoun.
Adnoun is formed from "ad-" (meaning to or toward) and "noun" (from Latin "nomen", meaning name). The term refers to a word that is added to a noun to modify or describe it, thus indicating a relationship to the noun.
Think of 'ad-' meaning 'to' or 'toward' and 'noun' meaning 'name' — this helps you remember that an adnoun is something that adds to or modifies a name.