The Collaborative International Dictionary of English v.0.48:

Yoga \Yo"ga\, n. [Skr. y[=o]ga union.]
   A species of asceticism among the Hindoos, which consists in
   a complete abstraction from all worldly objects, by which the
   votary expects to obtain union with the universal spirit, and
   to acquire superhuman faculties.
   [1913 Webster]

WordNet (r) 3.0 (2006):

yoga
    n 1: Hindu discipline aimed at training the consciousness for a
         state of perfect spiritual insight and tranquility that is
         achieved through the three paths of actions and knowledge
         and devotion
    2: a system of exercises practiced as part of the Hindu
       discipline to promote control of the body and mind

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "yoga":
   anagoge, anagogics, anthroposophy, athletics, breather, cabala,
   cabalism, calisthenics, constitutional, daily dozen, drill,
   esotericism, esoterics, esoterism, esotery, exercise, exercising,
   gymnastic exercises, gymnastics, hocus-pocus, isometrics,
   mumbo jumbo, mystery, mysticism, mystification, occultism,
   physical education, physical jerks, practice, setting-up exercises,
   stretch, symbolics, symbolism, workout, yogeeism, yogism