Answer: the extent of dissociation of the acid or base
Step-by-step explanation:
When an acid or base dissociates, it means that they go back to their original ions in order to be able to react with the atoms in the solution they dissociated in.
Stronger acids and bases are able to almost completely dissociate in a solution which is why they are able to react with the atoms in the solution very easily. The strength of an acid or base is therefore measured by how much dissociation they can manage in a solution.