From a cellular perspective, senescence has been considered a binary state, wherein cells are either senescent or not. This reductionist notion, often defined as irreversible growth arrest, has guided efforts to identify universal biomarkers and senolytics, but both have consistently eluded us. This outcome is not surprising, given that the biological nature of senescence may not be strictly irreversible; the accumulated evidence suggests that growth arrest can become unstable over time, with cells acquiring alterations, occasionally regaining proliferative capacity, or undergoing partial reprogramming, and exhibiting a heterogeneous spectrum of phenotypes (“senotypes”) influenced by tissue types, stressors, temporal dynamics, and disease states.