Degressive proportionality is a type of formula for electing the members of a legislature or other decision-making body. If a body is elected by a number of regions, states or other subdivisions, degressive proportionality means that while the subdivisions do not each elect an equal number of members, smaller subdivision are 'overrepresented' in terms of their population.
If the equal representation of subdivisions is used, then each subdivision elects the same number of members. On the other hand if apportionment on the basis of population is used, then each subdivision elects a number of members strictly proportionate to its population. Degressive proportionality means any intermediate method that is a compromise between these two approaches. As a term it does not describe any one particular formula.
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Under the Treaty of Lisbon, the European Parliament uses a system of degressive proportionality to allocate its 750 seats among the member states of the European Union. Treaty negotiations, rather than a specific formula, determine the apportionment between member states.
Any system that reserves a minimum number seats for a sub-body is to some extent degressively proportional. The most famous example is perhaps the election of the US presidential Electoral College. As each state has a minimum of three members of the college, smaller states such as Wyoming and Vermont effectively have disproportionally more say in the election than larger states, the extreme being California.