Yoga (Skt.): an ancient philosophical system of India included under the broad designation of Hinduism. Yoga is also a form of gentle physical training, Hatha Yoga, associated with Hindu and Buddhist Tantra. Yoga also refers to the two types of meditation practice in Tantric Buddhism, Generation Stage and Perfection Stage Yoga. Yantra Yoga refers to the physically aggressive training in Tantric Buddhism and Karma Yoga refers to the path using esoteric sexual practices as a path to enlightenment. Yoga is an extremely common term used in Vajrayana Buddhism and can be applied to almost any type of meditation or ritual practice.
Jeff Watt 12-2005 [updated 6-2017]
(The images below are only a selection of examples from the links above).