the.com/reflectivity

means A measure of how much light or radiation a surface bounces back rather than absorbing, usually expressed as a percentage or ratio.

from Built from 'reflect,' which traces to Latin reflectere — re- ('back') plus flectere ('to bend') — so the literal image is of light being bent backward rather than passing through or sinking in. The '-ivity' ending turns the verb into a measurable property, the same pattern that gives us 'sensitivity' or 'conductivity' — scientists love nouning a tendency so they can stick a number on it.