The Collaborative International Dictionary
Semidiurnal
Semidiurnal \Sem`i*di*ur"nal\, a.
Pertaining to, or accomplished in, half a day, or twelve hours; occurring twice every day.
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.
Wiktionary
semidiurnal
a. Twice daily (context usually of tides English).
Usage examples of "semidiurnal".
When the tides are semidiurnal, that means you get high tide at, say, six o'clock in the morning, low tide a little after noon, and another high tide at about six-twenty-five that night, and another low tide six hours and twelve minutes af-ter that.