Albedo
Etymology
From Latin "albedo" ("whiteness"), from "albus" ("white"). The physicist Johann Heinrich Lambert introduced the concept in 1760; astronomers and climate scientists use it to compare how much sunlight planets and surfaces reflect.
noun · /al-BEE-doh/
The fraction of light a surface reflects rather than absorbs.
What it names
A single number for how reflective something is — central to understanding climate, since bright ice bounces sunlight away while dark ocean and asphalt soak up heat.
Example
As the white sea ice melts, the ocean’s low albedo takes over, absorbing more heat and melting still more ice.
Related idea
Related to but narrower than reflectivity in general; albedo specifically means the ratio of reflected to incoming light, averaged over a whole surface, and drives feedback loops in the climate.