— Sigma
Lowercase sigma (σ) wears several hats: electrical conductivity, mechanical stress, and — as the Stefan-Boltzmann constant σ — the constant governing thermal radiation. Context always tells you which one is meant.
SI unit: S/m (as conductivity); Pa (as stress)
Used in
- Stefan-Boltzmann law P = σAT⁴
- Electrical conductivity
- Mechanical stress σ = F/A