Real Dictionary

yoga
uncountable noun: Yoga is a philosophy which first developed in India, in which physical exercises and meditation are believed to help people to become calmer and united in spirit with God.
uncountable noun: Yoga is a type of exercise in which you move your body into various positions in order to become more fit or flexible, to improve your breathing, and to relax your mind.

yoga in American English
a mystic and ascetic Hindu discipline by which one seeks to achieve liberation of the self and union with the supreme spirit or universal soul through intense concentration, deep meditation, and practices involving prescribed postures, controlled breathin
a school of Hindu philosophy advocating and prescribing a course of physical and mental disciplines for attaining liberation from the material world and union of the self with the Supreme Being or ultimate principle

yoga in British English
a Hindu system of philosophy aiming at the mystical union of the self with the Supreme Being in a state of complete awareness and tranquillity through certain physical and mental exercises