The Collaborative International Dictionary of English v.0.48:

Semidouble \Sem"i*dou`ble\, n. (Eccl.)
   An office or feast celebrated with less solemnity than the
   double ones. See Double, n., 8.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Semidouble \Sem`i*dou"ble\, a. (Bot.)
   Having the outermost stamens converted into petals, while the
   inner ones remain perfect; -- said of a flower.
   [1913 Webster]