HETP:

The column is assumed to be made up of a large number of plates where equilibrium is achieved. The plate height (HETP) is defined as that length of the column which will yield in equilibrium with mean concentration over that length in the stationary phase. In other words, equilibrium of the solute between the mobile and stationary phase is assumed to take place. Movement of solute down the column is then assumed to take place as stepwise transfer of equilibrated mobile phase from one plate to the next.