Source : Webster's Revised Unabridged Dictionary (1913)

Semidetached \Sem`i*de*tached"\, a.
   Half detached; partly distinct or separate.

   {Semidetached house}, one of two tenements under a single
      roof, but separated by a party wall. [Eng.]

Source : WordNet®

semidetached house
     n : a house with two units sharing a common wall [syn: {duplex
         house}, {duplex}]