Even though we might approach yoga for health benefits, originally the meaning of yoga has a more spiritual purpose
In recent times yoga has become popular throughout the Western world as a form of exercise and for it's potential health benefits, but keeping us fit and healthy are actually spin-off benefits of yoga and are not it's underlying purpose. Originally the meaning of yoga was to awaken us to our true nature (what we might think of as spiritual awakening) and this is still it's underlying purpose.
Yoga is a Sanskrit word meaning 'union' or 'yoking together'1 however what most of us understand by the term yoga is a system of physical techniques that originated in India and has since become popular worldwide. The most commonly known form of yoga in the West is probably Hatha Yoga, although there are other forms such as the Tibetan Yoga known as Yantra Yoga.
So if yoga means 'union' what is it exactly that is being 'united'? Our mind, body and spirit. Through yoga practice we cultivate awareness and increasingly become more present to our total experience.
In our everyday lives we tend to think of ourselves as separate from nature, and each other. We even tend to think of our own body and mind, heart and soul, personality and spirit etc. as separate things yet in essence they aren't really separate at all.
"Yoga is the settling of the mind into silence." - Patanjali, 200-150BCE
As we go about our daily business, our busy 'monkey mind' rushes here and there, grasping at this and that, always seeking the next thing to entertain and/or amuse itself. This creates an illusion of separation. Through our yoga practice this mind calms down and becomes more still.
In that stillness and silence we are able to observe how our mind works. How our thoughts rise and fall and, more importantly, that this all takes place within a background 'awareness' or presence. We notice that our 'presence' remains in the stillness (even when we are not thinking) and that this presence is a continuous thread in everything we do.
"Yoga is a Sanskrit word that in Tibetan is rendered as naljor. Yoga originally means ‘union’, but naljor really means knowledge of the primordial condition." - Chögyal Namkhai Norbu, Yantra Yoga Master
As we place more of our attention on our presence, eventually we realize, that it is what connects us, and everything. We notice that all our thoughts arise from this presence and fall back into it. Yoga is the realization that this 'presence' is who we are. It is the universal mind of oneness. When we realize that our awareness, or presence, permeates everything then we are whole.
When we see how everything was connected all along then we realize that 'union' was there all along, we just needed to become still in order to see it! The deeper meaning of yoga is the realization of being one with all that we do.
See Yoga at http://www.etymonline.com/index.php?allowed_in_frame=0&search=yoga&searchmode=none