A union can be between two things.

The two things have been taken by some as the body and mind,
or body-mind and the soul, or the individual soul and the cosmic being.

In the Bhagavad Gita, Chapter 6, verse 23, Lord Krishna says:

This means "May you know that Yoga is the separation of one’s union with sorrow", i.e. with the mind-body-sense complex. In other words, while "union" presumes two realities that need to be united, Advaita Vedanta teaches that ...

         "ekam eva advitīyam Brahman"
          All that is here is the One Infinite Reality, which is Who You Are.

It is that One reality which is apparently manifest as many. 
Therefore “union” is the result of the Self-discovery of the One reality, and at the same time disowning the apparent identifications of the Self with the body, mind, sense complex.

Yoga, a universal spiritual practise for mankind, helps purifying the body-mind-sense complex through asana, pranayama and meditation, it has the power to transform one’s life, completely - developing a deeper awareness, and thus preparing the mind for the knowledge of one’s already accomplished wholeness.

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  VEDANTA

  All that is here is the One Infinite Reality, which is Who You Are. It is that One reality which is apparently manifest as many. 

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  HATHA YOGA

  In Sanskrit, the primary definition of the term Yoga is the state of union with the Divine or the experience of oneness with ...

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  MEDITATION

  Meditation means awareness. Whatever you do with awareness is meditation. "Watching your breath" is meditation ...

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  AYURVEDA

  Ayurveda or ayurvedicmedicine is a system of traditional medicine native to India and a form of alternative medicine ...