A Saturn return happens when transiting Saturn returns to the exact zodiac degree it occupied at someone's birth — roughly once every 29.5 years, so around ages 28-30, again around 58-60, and (for the fortunate) around 88-90.
It's one of the few transits with genuine consensus across astrological traditions as a major life marker: a period of reckoning with structures, responsibilities, and commitments that were built (or avoided) in the prior cycle, often experienced as a pressured but ultimately clarifying transition into a more solid, self-defined version of adulthood.
This is a transit concept — it depends on where the planets are right now relative to a fixed natal chart, not something visible in the natal chart alone.