A plasma shields any charge perturbation over the Debye length λ_D = √(ε₀kT/ne²). Mobile electrons rearrange around a positive test charge, reducing the effective potential from φ∝1/r (bare Coulomb) to φ∝e^(−r/λ_D)/r (Yukawa/Debye-Hückel). Lower temperature or higher density → stronger shielding.