The opposite of a hydronium ion is a hydroxide ion (OH- ). Just like a strong acid, or a weak acid, bases are classified based on how easily they ionise in water.
Ammonia is an example of a weak base.
EXAMPLE:
NH3 ⇋ NH4+ + OH-
Like acids, the larger the Kb, the higher the ionisation of OH the stronger the base.
As we learn more about the relationship between acids and bases, you will be able to use the Ka value to determine Kb values for complimentary acid, base pairs.