“Niscintya yoga ucyate”- The thought-free state is yoga.
When does the mind attain freedom from thoughts? When Prana ceases, in other words on the cessation of Prana-dynamism when confluence with the still Prana is effected, this state is designated to be yoga. In this state—mind, thoughts etc. are all absent.
In short, the still state is yoga. Conversely union of the two is also termed as yoga. This also is an analogous statement, that is causation of mergence of dynamic Prana with still Prana.
Dynamic Prana originates from still Prana, hence the total creation.
Later when vibratory Prana attains confluence with Prana in statice then nothingness prevails. This state of nothingness is yoga.