This being a Theravada Buddhism forum, discussing Yoga and Tibetan Buddhism may not get you the answers you're looking for...most here will probably go with something closer to the definition below...I don't see 'unification' mentioned anywhere here...and, personally speaking, things pertaining to 'astral', at this point in time, are over my head...
Wishing you well... http://www.palikanon.com/english/wtb/s_t/samaadhi.htm
'concentration'; lit. 'the (mental) state of being firmly fixed' (sam+ā+Ö hā),
is the fixing of the mind on a single object.
"One-pointedness of mind (cittass' ekaggatā), Brother Visakha, this is called concentration" (M. 44).
Concentration - though often very weak - is one of the 7 mental concomitants inseparably associated with all consciousness. Cf. nāma, cetanā.
Right concentration (sammā-samādhi), as the last link of the 8-fold Path (s. magga), is defined as the 4 meditative absorptions (jhāna). In a wider sense, comprising also much weaker states of concentration, it is associated with all karmically wholesome (kusala) consciousness. Wrong concentration (micchā-samādhi) is concentration associated with all karmically unwholesome (akusala) consciousness. Wherever in the texts this term is not differentiated by 'right' or 'wrong', there 'right' concentration is meant .
In concentration one distinguishes 3 grades of intensity:
(1) 'Preparatory concentration' (parikamma-samādhi) existing at the beginning of the mental exercise.
(2) 'Neighbourhood concentration' (upacāra-samādhi), i.e. concentration 'approaching' but not yet attaining the 1st absorption (jhāna), which in certain mental exercises is marked by the appearance of the so-called 'counter-image' (patibhāga-nimitta).
(3) 'Attainment concentration' (appanā-samādhi), i.e. that concentration which is present during the absorptions. (App.)