What are theoretical plates in HPLC?

Theoretical plates are a measuring tool of HPLC column efficiency. A theoretical plate is a hypothetical zone or stage in which two phases, the stationary phase and the liquid mobile phase in the case of HPLC, establish an equilibrium with each other. The more theoretical plates available within a column, the more equilibrations between the stationary and mobile phases are possible, which leads to a separation with better resolution.