1.  From the Sanskrit “yug” meaning union, yoga means a union between a human and the Divine. There are several different types of yoga. Most people equate it with hatha (Sanskrit “hat” “ha” meaning sun-moon), which involves moving the body into a series of positions known as asanas. Today, many people enjoy hatha yoga for its physical benefits, although when accompanied by certain muscle tension or “locks,” breathing techniques, and visualizations, it can provide more spiritual development. Other types of yoga include:

Karma yoga: The yoga of action. Practitioners do what is right without a focus on the results of their actions.
Bhakti yoga: The yoga of devotion. Practitioners live in devotion to a deity.
Jnana yoga: The yoga of knowledge. Practitioners become closer to the Divine through the study of spiritual texts.
Tantric yoga: Using Tantric techniques, including mantra, yantra, breathwork, and sexual activity to become closer to the Divine.
Kundalini yoga: Popular name for Laya yoga, the use of visualization and breathwork to generate and move kundalini energy up an energy path associated with the spine, and through the chakras, opening them and resulting in enlightenment.
Raja yoga: The royal yoga. Practitioners use meditation to become closer to the Divine.

3.  Sanskrit word meaning “Union, to Yoke”: An Indian system, like Taoism and Western Magick, intended to develop the whole person including the psychic and spiritual bodies. The Sanskrit root not only a implies a linking with cosmic forces but also suggests the harnessing (a yoke is a harness) and control of our own energies. Yoga is a total art and science of living leading to “skill in action.”

The Sanskrit root "yug" gives rise to the following meanings:

I. Yoking, team; vehicle; equipment (of an army).

1. Performance, employment; occupation.
2. Use, application, method.
3. Remedy, cure, wholeness.
4. Means, device, instrument producing a result.
5. Spell, magic, dextrous feat.
6. Opportunity, undertaking, task performed perfectly.
   1. Union, contact with, relationship.
   2. Combination, mixture, bringing together of polarities or complementaries (e.g., arrow with target, key with lock).
   3. Aquisition of, gain, profit (alchemy).
   4. Order, succession, correctness.
   5. Aggregate, sum, conjunction (of stars); constellation.
   6. Fitness, propriety, strenuousness, extertion, endeavor, zeal, assiduity.
   7. Mental concentration, systematic abstraction, system of philosophy.
   8. Unity of soul (Purusha) and Nature (Prakrit) (alchemical marriage).
   9. Connection of a word with its root, etymological meaning of a word, deriving one word from another.

Suggested Reading:

Muni: Yoga: The Ultimate Spiritual Path

Mumford & Riddle: Yoga Nidra: Chakra Theory

4.  Yoking or union; an Indian spiritual tradition closely related to Tantra. While most Westerners think of Yoga as a form of exercise characterized by various asanas (postures), there are many ways to practice Yoga, and some of them do not involve physical postures at all. It can be useful to think of Yoga as a state of mental stillness, accompanied by a feeling of union or merger with all that is.