YS 1.2

The Yoga Sutras attributed to Patanjali, offer any student of yoga a lot of insight into the workings of the mind. One particularly insightful sutra comes in the first chapter, verse two. The sutra states "yogas chitta vritti nirodhah" and gives us a definition of yoga.

The Yoga Sutras are about the mind and not about a physical practice. So in this case, the first word of this sutra, yoga, actually refers to samadhi, a very advanced mental practice. The YS as a whole take into account that the student has already done an immense amount of practice and study, and has already practiced meditation at length. Therefore, when we read or hear about the YS, it is slightly misguided to attribute the teachings to asana (posture) practice. The "yoga" of the YS is mental.

The second word in YS 1.2 is chitta. This is what the mind is made up of. You can think of this as the actual makings of your mind.

Vritti is a very important word. Vrittis are thought waves. They are fluctuations that occur within your chitta, or your mind. Every thought you have, every single disturbance or fluctuation that happens in your mind is a vritti.

Nirodhah refers to the restraint of the vrittis, or thought waves. This is also commonly understood as the cessation of thought waves.

Put all of the words together and YS 1.2 can be understood as: 
Yoga (samadhi) is the restraint of the thought waves in the mind

This is the definition of yoga as it is in the Yoga Sutras.