VO2 max (also maximal oxygen consumption, maximal oxygen uptake or maximal aerobic capacity) is the maximum rate of oxygen consumption attainable during physical exertion.

Useful links:

https://www.runnersworld.com/uk/training/a773975/what-is-running-economy/

https://www.healthline.com/health/vo2-max

The Effect of Training Intensity on VO2max in Young Healthy Adults: A Meta-Regression and Meta-Analysis - PMC (nih.gov)

Survival of the fittest: VO2max, a key predictor of longevity? - PubMed (nih.gov)