My favorite definitions:
samsara, (Sanskrit: “flowing around”) in Indian philosophy, the central conception of metempsychosis: the soul, finding itself awash in the “sea of samsara,” strives to find release (moksha) from the bonds of its own past deeds (karma), which form part of the general web of which samsara is made.
The word samsara is based on a verbal root with the sense “to flow” and means the “flowing-on” of the stream of consciousness from one moment to the next and from one lifetime to another. The Buddha is recorded as saying that the origin of this process is “beyond reckoning,” or fundamentally unknowable.