Malapportionment is broad and systematic variance in the size of electoral constituencies (at least within electoral systems which have them). It is a tendency for the size of constituencies to vary according to some factor such as geographic location. Well-known examples include the differences between urban and rural constituency sizes in many Australian states (currently Western Australia, though Queensland and South Australia in the past afforded far more notorious examples), and the recently abolished smaller United Kingdom parliamentary constituencies in Scotland. The UK retains a substantial malapportionment in favour of urban voters, which currently benefits the British Labour Party. The effects of malapportionment vary with time: deliberate overrepresentation of rural Queensland changed from favouring Labor to favouring the National Party.

In effect, the value of votes in one or more constituency (districts or ridings) will differ from that in one or more other constituencies. Malapportionment is possible only in electoral systems with more than one electoral constituency. Thus a proportional representation electoral system with only one national constituency like those in Israel and the Netherlands cannot be malapportioned.

Another example is the systematic over-representation of voters in more rural prefectures and under-prepresentation of voters in more urban prefectures in Japanese parliamentary elections. The conservative Liberal Democratic Party thus wins more seats in the Japanese parliament because its voters are concentrated in more rural prefectures.