adj

• (of a person) Not showing emotion; calm and in control of oneself.
  "Be cool. There's no need to panic."
• (informal, of a person) Knowing what to do and how to behave; behaving with effortless and enviable style and panache; considered popular by others.
  "[…] while the coachman holding whip and reins in one hand, takes off his hat with the other, and resting it on his knees, pulls out his handkerchief, and wipes his forehead, partly because he has a habit of doing it, and partly because it's as well to shew the passengers how cool he is, and what an easy thing it is to drive four-in-hand, when you have had as much practice as he has."
• (informal, originally African-American Vernacular) Fashionable; trendy; hip.
  "2008, Lou Schuler, "Foreward", in Nate Green, Built for Show, page xii The fact that I was middle-aged, bald, married, and raising girls instead of chasing them didn't really bother me. Muscles are cool at any age."
• (informal) All right; acceptable; good.
  "Is it cool if I sleep here tonight?"

verb

• (intransitive, literally) To lose heat, to get colder.
  "I like to let my tea cool before drinking it so I don't burn my tongue."
• (transitive, literally) To make cooler, less warm.
  "Send Lazarus, that he may dip the tip of his finger in water, and cool my tongue."
• (intransitive, figuratively) To become less intense, e.g. less amicable or passionate.
  "Relations cooled between the USA and the USSR after 1980."
• (transitive, figuratively) To make less intense, e.g. less amicable or passionate.
  "We have reason to cool our raging motions, our carnal stings, our unbitted lusts."

name

• (programming) Initialism of CLIPS Object-Oriented Language.