semidiurnal

(sĕmˌē-dī-ûrˈnəl, sĕmˌī-)

1. Of, relating to, occurring, or performed during half a day.
2. Occurring or coming approximately once every 12 hours, as the tides.
3. Of or relating to the arc described by a celestial body between its meridian passage and its points of rising or setting.