From The Collaborative International Dictionary of English v.0.48 :
Semidetached \Sem`i*de*tached"\, a.
Half detached; partly distinct or separate.
[1913 Webster]
Semidetached house, one of two tenements under a single
roof, but separated by a party wall. [Eng.]
[1913 Webster]
From WordNet (r) 3.0 (2006) :
semidetached house
n 1: a house with two units sharing a common wall [syn: duplex
house, duplex, semidetached house]