A highest-averages formula for allocating seats in proportional systems: each party's votes are divided by 1, 2, 3 and so on, and seats go to the largest resulting quotients.
In more detail
Devised by the Belgian lawyer Victor D'Hondt in 1878 and adopted for Belgian national elections in 1899, the method is used today in many list-PR systems, including Spain, Portugal and elections to the European Parliament in several member states. It gives a mild advantage to larger parties compared with the Sainte-Laguë method, an effect that grows when districts are small. It is mathematically equivalent to the Jefferson method used for apportionment in the United States.