An adjective was a word used to describe a noun.
In 2370, Data found "strange" to be an insufficient adjective to describe his dream. (TNG: "Phantasms")
In 2375, One recognized the word "volatile" as an English adjective after Seven of Nine used it to describe B'Elanna Torres's temperament. (VOY: "Drone")
In 2376, "Vaadwaur" was defined as an "archaic adjective" in the Talaxian language by USS Voyager's computer. (VOY: "Dragon's Teeth")