Gerontology: A branch of biology focusing on the common mechanisms of aging across all multicellular species. Gerontologists, for example, are keen to understand species that appear to exhibit very gradual or negligible senescence over a long time interval. In this context, gerontologists may study yeast, worms, fruit flies, mice, rock fish, tortoises, bats, parrots, humans, and other creatures exhibiting exceptional longevity.