[sem-ee-dahy-ur-nl, sem-ahy-]
pertaining to, consisting of, or accomplished in half a day.
occurring every 12 hours or twice each day.

1585–95; semi- + diurnal Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
semidiurnal (ˌsɛmɪdaɪˈɜːnəl)
1.  of or continuing during half a day
2.  occurring every 12 hours

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Example sentences
Most tides are semidiurnal, which means they take place twice a day.
The littoral cell features a semidiurnal tidal pattern, two high tides and two
  low tides of unequal height every day.
The zero-dimensional model consists of a well-mixed control volume subjected to
  a semidiurnal tide, with a cohesive sediment bed.
Even though total coliform has a day-night cycle, it shows a stronger
  relationship to the semidiurnal tidal cycle.
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