Adrenalin is a chemical your body makes when you are afraid, angry, or suddenly excited. It makes your heart beat faster, sharpens you, and gets you ready to run or fight — the old “danger” setting. Doctors and biology teachers mean that chemical. Though actually, in everyday talk people stretch it a long way: “I did it for the adrenalin” often just means “for the thrill,” and “adrenalin rush” is the jolt you feel on a roller coaster or before you walk on stage. That loose use is extremely common; it is not a lab measurement. You will also see the spelling adrenaline, with an e on the end. In speech they sound the same; the e-spelling is now the usual one in many books. An older trademark used the shorter form.