The word aerobic is an adjective. Aerobic means (1) depending on free oxygen or air, (2) based on or using the principles of aerobics; enhancing respiratory and circulatory efficiency.
The following articles and web pages elaborate on the word aerobic.
Elliptical machines combine the motion of stair climbers with that of treadmills, and are a popular way to get an aerobic workout indoors. Elliptical trainers provide a low-impact workout.
Elliptical trainers provide a low-impact full-body cardiovascular workout. They are a popular choice for an indoor aerobic workout, but without the impact of jogging on a treadmill or pedaling a stair stepper.
An exercise or stationary bike is an effective way to get an aerobic workout indoors. It is ideal for when the weather is bad, and it can be used by cyclists for training during the off-season.
The following digital image illustrates the word aerobic.