A strong base has a large concentration of OH-. A typical example of this is NaOH.
Also a strong base usually has a very low concentration of H^+ ions and a very high concentration of OH^- ions.
Lastly, it must have a very high base dissociation constant. Its Kb, the base dissociation constant is a measure of how completely a base dissociates into its component ions in water. The higher the Kb, the stronger the base. A strong base dissociates completely in solution.
