cardio: A term (short for cardiovascular) that is often used interchangeably with aerobic. Aerobic exercise is any repetitive activity that you do long enough and hard enough to challenge your heart and lungs.

chi: Otherwise known as "life energy," this is the life force that pulses through your body and keeps you vital. Blocked chi can cause sickness or unhappiness.

circuit training: A fast-paced class or exercise routine in which you do one exercise for 30 seconds to 5 minutes and then move on to another exercise at the next station. Combines cardio exercise with strength training.

core: The abdomen, obliques, lower back, butt, and so on, that form the midsection. Many forms of exercise focus on strengthening the body's core.

core conditioning: A non-aerobic, muscle-toning class, usually focused on core strength.

cross-training: A method of varying your workouts to take your fitness to the next level by adding new forms of training to your current routine.