Congressional reapportionment refers to the process by which seats in the House of Representatives are reallocated after every constitutionally mandated decennial census using a complex mathematical population formula that ensures the distribution is even. Every state is apportioned a number of seats based on the aggregate population and is guaranteed at least a single seat by the constitution regardless of population.
The term congressional reapportionment is different from redistricting, which refers to the division of congressional districts after every state has been apportioned. Both redistricting and reapportionment impact the local state directly. Reapportionment also affects the number of electors every state receives for presidential elections.