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]

WordNet (r) 3.0 (2006):

semi-detached house
    n 1: a dwelling that is attached to something on only one side