The definition of Yoga is a long and complex answer.
The answer will vary based on who you ask and their personal experience with it. To some people yoga is the exercise class they take to compliment their workout. To others yoga is a philosophy for living.
Most yoga classes are asana classes with a little bit of breathing and meditation built in. Asana is really a very small portion of what yoga is overall. The history and original texts that form the modern basis of yoga is NOT light reading, but a good place to start is the wiki pages. You can read more about the 8 limbs of yoga on this wiki article.
Some believe the father of yoga was a sage named Patanjali who wrote his treatise on yoga:
http://en.wikipedia.org/wiki/Yoga_Sutras_of_Patanjali
Sutra 1.2 is my personal definition that I hold in my heart:
Yoga is the stilling of the activities of the mind.
Yoga as a whole is sometimes refered to Ashtanga Yoga or the 8 Folded Path. Others use the 8 Limbs of Yoga.
| Limb of Yoga | Sanskrit Name | Root Words Breakdown | Description |
| 1. Yama | यम (Yama) | य (Ya) = Control, Ma = Restraint | Ethical standards and sense of integrity, focusing on our behavior and how we conduct ourselves in life. The five yamas are Ahimsa (non-violence), Satya (truthfulness), Asteya (non-stealing), Brahmacharya (continence), and Aparigraha (non-covetousness). |
| 2. Niyama | नियम (Niyama) | नि (Ni) = Inward, यम (Yama) = Discipline | Self-discipline and spiritual observances. The five niyamas are Saucha (cleanliness), Santosha (contentment), Tapas (heat; spiritual austerities), Svadhyaya (study of sacred scriptures and of one’s self), and Ishvara Pranidhana (surrender to a higher power). |
| 3. Asana | आसन (Āsana) | आ (Ā) = To sit, स (Sa) = With, न (Na) = Seat | Physical postures. Initially intended to prepare the body for sitting, leading to meditation by developing the habit of discipline and the ability to concentrate. |
| 4. Pranayama | प्राणायाम (Prāṇāyāma) | प्राण (Prāṇa) = Life force, आयाम (Āyāma) = Extension | Breathing techniques designed to control the life force (prana) within us. |
| 5. Pratyahara | प्रत्याहार (Pratyāhāra) | रति (Prati) = Against or away, आहार (Āhāra) = Food or intake | Withdrawal from the 5 senses. This means turning the focus inward and away from the external world and its distractions. |
| 6. Dharana | धारणा (Dhāraṇā) | धार (Dhāra) = Hold or concentrate, णा (Na) = State or condition | Concentration. This practice involves intense focus on a single point or object. |
| 7. Dhyana | ध्यान (Dhyāna) | ध्य (Dhya) = Meditate or contemplate, न (Na) = State or condition | Meditation. This practice is an uninterrupted flow of concentration, aimed at quieting the mind and achieving a deep state of contemplation. |
| 8. Samadhi | समाधि (Samādhi) | सम (Sama) = Together or integrated, धि (Dhi) = Mind | State of ecstasy or bliss. This is the ultimate goal of yoga, where the practitioner transcends the self and experiences unity with the divine or universal consciousness. |