Definition - A word that modifies the meaning of a noun or a pronoun.

Example -
The red truck
(In the above, the word red is an adjective and the word truck is a noun.)

Etymology -
The word derives via Old French from the Latin adjectivum, that which is added. (from the Latin ad, to + jacere, to throw).

Oxford English Dictionary -
Its first citation is from 1414:
"Scotland is like a noun adjective that cannot stand without a substantive."
(Dk. of Exeter to Henry IV in Hall Chron. (1809), 55)

Please comment