Cardio is exercise that gets your heart working hard for a stretch of time — running, swimming, cycling, a dance class, skipping. Gyms and trainers use it as a noun (“I did thirty minutes of cardio”) and sometimes in front of another word (“a cardio workout”). It is informal fitness English, cut down from cardiovascular, which means “about the heart and the blood vessels.” People contrast it with weights or strength training: one is for the heart and lungs, the other is for muscle. It is not a doctor’s word for heart disease; cardiac lives in that hospital world. At the gym, cardio just means the sweaty, breathing-hard kind of exercise.