When viscoelasticity is modeled in the frequency domain, it will act as a loss factor damping. The complex modulus G*(ω) is the frequency-domain representation of the stress relaxation function of viscoelastic material. It is defined as

where G' is the storage modulus, G'' is the loss modulus, and their ratio ηs = G''/G' is the loss factor. The term G' defines the amount of stored energy for the applied strain, whereas G'' defines the amount of energy dissipated as heat; G', G'', and ηs can all be frequency dependent.