The Collaborative International Dictionary of English v.0.48:

Aerobics \Aer*o"bics\, n. pl., but sing. or pl. in usage
   1. physical exercises performed in a sustained and strenuous
      fashion designed to cause a temporary increase in
      respiration rate and heart rate, such a running, swimming,
      walking, or calisthenics. Calisthenic sessions in groups,
      performed by following a leader in moving to a rhythmic
      musical accompaniment, are a popular form of aerobics.
      [PJC.]

   2. a system of physical conditioning relying largely on
      aerobics exercises.
      [PJC.]

WordNet (r) 3.0 (2006):

aerobics
    n 1: exercise that increases the need for oxygen [syn:
         aerobics, aerobic exercise]