'round of rebirth', lit. perpetual wandering', is a name by which is designated the sea of life ever restlessly heaving up and down, the symbol of this continuous process of ever again and again being born, growing old, suffering and dying.
More precisely put, samsāra is the unbroken chain of the five-fold khandha-combinations, which, constantly changing from moment to moment follow continuously one upon the other through inconceivable periods of time.
Of this samsāra, a single lifetime constitutes only a tiny and fleeting fraction; hence to be able to comprehend the first noble truth of universal suffering, one must let one's gaze rest upon the samsāra, upon this frightful chain of rebirths, and not merely upon one single life-time, which, of course, may be sometimes less painful. -
Cf. tilakkhana, anattā, paramattha, patisandhi.