Final answer:
The Stewart approach refers to the 'strong ion difference' as the parameter for concentration of completely dissociated ions in a solution.
Step-by-step explanation:
In the Stewart approach, which is a physicochemical approach to acid-base balance, the parameter representing the concentration of completely dissociated ions in a solution is known as the strong ion difference (SID). This concept is not directly tied to the equilibrium constant of an acid or the acid dissociation constant (Ka) used in other acid-base models, however, it relies on the principle that strong acids and bases in blood plasma are completely dissociated at physiological pH. The Stewart approach considers how factors like SID, the total weak acid concentration, and the partial pressure of carbon dioxide (pCO2) influence hydrogen ion concentration and thus, the pH of blood.