Overhang seats are constituency seats won in an election under the traditional mixed member proportional system, when a party's share of the nationwide votes would entitle it to fewer seats than the number of individual constituencies won. Wikipedia

A constituency seat in excess of a party's entitlement Wikipedia Disambiguation

A legislative seat distribution, used to guarantee that a party gets their fair share of seats, when a party is entitled to fewer seats as a result of party votes than it has won constituencies. Additional seats granted to that party would be overhang seats. Wiktionary

Legislative seat distribution principle in certain countries. Wiktionary (translation)