A very good definition of yoga

http://www.swamij.com/yoga-note.htm

Traditionally, Yoga (Sanskrit: union) has referred to the realization through direct experience of the preexisting union between Atman and Brahman, Jivatman and Paramatman, and Shiva and Shakti, or the realization of Purusha standing alone as separate from Prakriti. Yoga is the realization  of union between the microcosm of individuality with the macrocosm of universality.

Yoga is the union of the
- Microcosm of individuality and the
- Macrocosm of universality

Yoga is the union of the
- Atman (Center of consciousness, Self; Vedanta) and
- Brahman (Absolute reality; Vedanta)

Yoga is the union of the
- Jivatman (Soul as consciousness plus traits; Vedanta) and
- Paramatman: (Self/soul as only consciousness; Vedanta)

Yoga is the union of
- Shiva (Static, latent, unchanging, masculine; Tantra) and
- Shakti (Active, manifesting, changing, feminine; Tantra)

Yoga is the dis-union of
- Purusha (Untainted consciousness; Sankyha-Yoga) and
- Prakriti (Primordial, unmanifest matter; Sankyha-Yoga)