1  definition  found 
 
  From  Webster's  Revised  Unabridged  Dictionary  (1913)  [web1913]: 
 
  Semidiurnal  \Sem`i*di*ur"nal\,  a. 
  1.  Pertaining  to  or  accomplished  in  half  a  day  or  twelve 
  hours;  occurring  twice  every  day 
 
  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. 

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