Yoga is a way of moving into stillness in order to experience the truth of

 who you are. It is also a way of learning to be centered in action so

that you always have the clearest perspective on what’s happening and

are therefore able to respond most appropriately. It is an ancient process

designed to help you uncover and discover your true nature so you can live

daily life with new awareness. 

Erich Schiffmann - 'Yoga The Spirit and Practice of Moving Into Stillness'

Yoga is a method for restraining the natural turbulence of thoughts,

which otherwise impartially prevents all men, of all lands, from glimpsing their

true nature of Spirit. 

Paramahansa Yogananda - 'Autobiography of a Yogi'

Coming together to unit to tie the strands of the mind together to attain

what was previously unattainable. Yoga has its roots in Indian thought

but its content is universal because it is about the means by which we can make

the changes we desire in our lives.

T.K.V. Desikachar - 'The Heart of Yoga'

Yoga dignifies the essential unity that is the basis of life. Like love, this great

truth embraces the full spectrum of reality, from the most spiritual to the most

material. In the highest sense, yoga, as spiritual union, signifies the soul’s union

with God. At the opposite end of the spectrum it embraces the affinity of atomic

particles for one another. Like love, moreover, the deeper reality implied is always

spiritual, even when treating of seemingly inanimate matter. 

Sri Kriyananda 

Yogascittavrttinirodhah Yoga is the ability to direct the mind

exclusively towards an object and sustain that direction

without any distractions. 

1-2 Patanjali’s Yogasutras T.K.V Desikachar translation

When the five senses and the mind are still, and reason itself rests in silence,

then begins the Path supreme. This calm steadiness of the senses is called Yoga. 

Then one should become watchful, because Yoga comes and goes.     

Katha Upanishad part 6