Cardio/ˈkɑːrdiəʊ/ Definition 1 of 2Pro Tip 1/3Exercise Context Used to describe a workout focusing on increasing heart rate and endurance. My trainer designed a cardio routine to improve my stamina. Pro Tip 2/3Informal Usage 'Cardio' is casual, ideal for conversations about fitness and health, not formal reports. She prefers cardio over weight lifting. Pro Tip 3/3Variety of Activities Covers any activity boosting heart rate, like running, cycling, or swimming. He switched to swimming for his daily cardio.