Samadhi sarva tatvAnAm samAvastha nirudyamatvam anAyAsa sthithimatvam iti samAdhi.

Union is the state of the concord of all principles. Union is without cause, without exertion, permanent and natural.

2.39 Siddha siddhAnta paddhatiH
of GorakshanAtha

Samadhi, Union
Samadhi is the experience of union. It is the aim of Yoga. Samadhi is described as a natural and spontaneous, (sahaja) holistic and undivided (alakh) stainless (niranjan) state of body and mind. It is not dependant on anything else (NirAlamba) and therefore  arises free of one´s own exertion in a natural way. 
Samadhi is nothing that can  be gained by activities since it is inborn (sahaja). It is a harmonious, realaxed, satisfied and blissful state, that is a natural predisposition of every being. Since it is nothing that can be achieved by effort, it arises on its own when the obstacles (kleshas) and impurities (malas), the causes of difficulties, sorrow, restlessness and suffering, are terminated.

The preceding five limbs of Yoga asana, pranayama, pratyahara dharana and dhyana serve to gradually dissolve the diverse traumatic imprints, impurities,  and past conditionings of our body, speech and mind so that the innate stainless nature of the body and mind can emerge.