semidiurnal

(-dī ʉrnəl)

adjective

  1. of, lasting, or performed in half a day
  2. occurring twice a day, as the tides

See semidiurnal in American Heritage Dictionary 4

adjective
  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.

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