In Electrodynamics, this tensor is defined in terms of the Maxwell Tensor as follows:

T^μν = (1/μ₀)(F^μρF^ν_ρ-(1/4)η^μνF^ρσF_ρσ)

For a fluid with pressure p and density ρ,

T^μν = (ρ+p)u^μu^ν + pg^μν

Together these act as the source for the gravitational field in General Relativity.