In short
Lifespan is the total number of years a person lives; healthspan is the portion of that lifespan spent free of significant chronic disease, disability, or functional decline.
The distinction matters because these two measures can move independently: modern medicine has substantially extended average lifespan over the past century, largely through reductions in infectious disease mortality and improvements in acute and chronic disease management, but a meaningful share of added years in many populations are spent living with one or more chronic conditions, a gap sometimes referred to in public health literature as "the morbidity-mortality gap" or the compression (or expansion) of morbidity.
This is why most contemporary longevity medicine and research explicitly frames its goal as extending healthspan — or ideally compressing morbidity into a shorter period at the very end of life — rather than simply extending total years lived regardless of health status. It is also a more practically achievable and individually meaningful goal: interventions like exercise and management of cardiometabolic risk factors have strong evidence for improving functional health and reducing disease burden in the years lived, even where lifespan-extension evidence in humans is inherently harder to generate (requiring decades-long trials).
In practice, most of the biomarkers and outcomes this glossary and the broader longevity field track — VO2 max, frailty measures, cardiometabolic markers, cognitive function — are healthspan measures, not direct lifespan measures, which is an important framing to keep in mind when evaluating any specific claim: an intervention that improves a healthspan-relevant biomarker has not thereby been shown to extend lifespan, even though the two goals are related and often pursued together.