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]