Emissivity
The dimensionless ratio of thermal energy radiated by a surface to that radiated by an ideal blackbody at the exact same temperature.
Governing formula
ε = E_surface / E_blackbody (0 ≤ ε ≤ 1)
SI unit
Dimensionless
In depth
According to Kirchhoff's law of thermal radiation, emissivity equals absorptivity ($ε = α$) at thermal equilibrium.
Examples in the real world
Polished silver having low emissivity ($ε ≈ 0.02$, reflecting heat), compared to matte black carbon soot ($ε ≈ 0.98$, emitting/absorbing heat).