The Collaborative International Dictionary of English v.0.48:

Semidiurnal \Sem`i*di*ur"nal\, a.
   1. Pertaining to, or accomplished in, half a day, or twelve
      hours; occurring twice every day.
      [1913 Webster]

   2. Pertaining to, or traversed in, six hours, or in half the
      time between the rising and setting of a heavenly body;
      as, a semidiurnal arc.
      [1913 Webster]