The steady state concentration of a drug is achieved when the rate of drug administration equals the rate of drug elimination, resulting in a constant plasma concentration during repeated dosing. This balance ensures that the drug's therapeutic effects are maintained without accumulation or sub-therapeutic levels. It is not the peak concentration after a single dose, nor the lowest concentration before the next dose, and it does not refer to concentrations after drug withdrawal. Understanding steady state is critical for dosing regimens to maintain efficacy and safety.

Reference: Goodman & Gilman's The Pharmacological Basis of Therapeutics, Brunton et al., 13th Edition.