Two attached dwellings on the same site. If the two attached dwellings are on separate title they are better known as semi detached. The term has been used extensively in the eastern suburbs of Sydney to describe 2 units where one is above and the other is below in a 2 storey building. A typical early 20th century duplex in Sydney (usually called a semi detached cottage) is shown below:
1912 single storey duplex
1990s 2 STOREY DUPLEX