The Avro Lancaster is a British Second World War heavy bomber. It was designed and manufactured by Avro as a contemporary of the Handley Page Halifax, both bombers having been developed to the same specification, as well as the Short Stirling, all three aircraft being four-engined heavy bombers adopted by the Royal Air Force (RAF) during the same era.

Heavy bombers are bomber aircraft capable of delivering the largest payload of air-to-ground weaponry and longest range of their era. Archetypal heavy bombers have therefore usually been among the largest and most powerful military aircraft at any point in time. In the second half of the 20th century, heavy bombers were largely superseded by strategic bombers, which were often even larger in size, had much longer ranges and were capable of delivering nuclear bombs.