Much bigger problem is to find one definition of Yoga accepted by all. We can start with:
The following are some relevant definitions of Yoga:
Yoga is the science of the self and can also be termed the science of man in depth, the science of conscious evolution or the science of human possibilities.
Yoga does not have only the concepts but also the tools and technology needed to find our self.
Yoga is an experiential science of peace, harmony, love friendliness and cooperation. Yoga is a science of awareness.
We have also traditional definition, but there are some problems about understanding it in modern world:
Yoga definition as given by the Yoga Sutras of Patañjali
Yoga Sutras of Patañjali #1.1:
Yoga Definition: Atha yoga anushasanam: Now (after having finished or let go of the preparation ,or primary, stages in life) the practice of yoga begins.
This is the first of all Yoga Sutras and it the has two primary purposes. The first purpose is to point out that everything that follows (the sutras) will define and explain the practice of yoga; yoga definition. The second purpose is to point out that in order to sincerely start and have the highest success with the path towards enlightenment or Self Realization, one has to be ready.
You have to be ready: not in the sense of eating a healthy meal, having enough sleep, preparing your meditation room or putting on some music etc. It means that you have to be ready in the highest sense of the word; ready to begin with yoga, the path towards Self Realization. Ready in the sense of being earnest enough to dedicate yourself to this practice and being willing to let go of anything that holds you down. Yoga definition says that to be truly ready you'll have to be finished or let go of doing the things you wanted to do in a social perspective. The preparation-stages of your life (childhood, study, parenting etc) should feel complete, finished, or unnecessary to the extent that you are ready to let go of it.
Only with this kind of readiness will the actual purpose of yoga definition (following in sutra #1.2 and #1.3) have a chance to unfold itself to the student. I am not saying that it cannot be done otherwise, but it will be a much slower process and the eventual goal of enlightenment will probably not be attained in this life time. Regular progress in the spiritual field can be made at any stage of life, but becoming the Buddha requires a high level of readiness. The inner decision to be ready can arise at earlier stages in life before one enters the stage of parenting or even the stage of studying. This depends on many factors in regard to the evolution of the individual.
Yoga Sutras of Patañjali #1.2:
Yogash chitta vritti nirodhah: Yoga is the restraint (control, mastery) of the modifications (changes, movement, thought-forms) of the mind field.
Sutra #1.2 is the most fundamental of the yoga sutras; it is the yoga definition. It defines Yoga, Union, Self realization, Mind Control and Enlightenment.
There are many words one can use to translate Nirodha, though none of them can fully comprehend the meaning of the sanskrit word. Nirodha defines a state of experience rather than an intellectual understanding. Roughly nirodha means control or restraint. In the context of this sutra we should understand it as that state I call Mind Control in which the mind field and all thought-forms (content) are fully under the control of consciousness. Also see: Consciousness and the mind.
How to attain this state of Nirodha is the primary subject of relevant website and it is also explained later on in the Yoga Sutras. When this state of Mind Control is attained, there follows an obvious result. Nirodha and Mind Control are terms that define the practice, the action, the establishment. Self Realization, Yoga and Enlightenment are terms that define the result that flows from this establishment. Both the action and the result are inseperatable.
The result of Mind Control or Nirodha is defined in the next sutra: Sutra #1.3. Sutra #1.2 and #1.3 are the two sutras defining the practice of yoga and the result gained from that practice.
Yoga Sutras of Patañjali #1.3:
Tada drashtuh svarupe avasthanam: Then the seer abides (and rests) in its own true and fundamental Self.
This yoga definition sutra describes the state that is the result of Nirodha or; Mind Control. What happens is that the seer, the observer, the witness, consciousness, remains dependent only on its own essence. It is at peace and resides in itself. This observer is free. It is no longer dependent and identified with the transitory shapes and forms of nature and the mind. This consicousness is no longer bound to creation; consciousness is liberated. You are now in the world but you are not of the world. This, is Yoga.Yoga Sutras of Patañjali #1.4:
Vritti sarupyam itaratra: At other times, the seer identifies with (is lost in) the modifications (thought-forms) of the mind field. When we are not residing in the state of pure consciousness as stated in yoga definition sutra #1.3, we must be residing in some other state of beingness. These states can differ considering the different levels of manifestation and the mind, but they are all within the mind. They are all forms, shapes, creations; not consciousness, which is formless and unmanifested. When we appear to take on the form of our thought-forms, we are unconscious of our Self, our consciousness. When consciousness is not aware of itself, it is subject to illusion, or maya. Consciousness then appears to take on the form of these objects of creation (i.e. body, a self-image, a desire, etc.) and mistakenly beliefs itself to be one or all of those things. This is why the question:"Who am I?" plays such an important role in the journey towards Self Realization. Once we find out the true answer to this question; not by intellectually saying: "I am this", or "I am that" but by truly realizing it within, we are enlightened.