Webster's English Dictionary

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directional

di.rec.tion.al \-shn*l, -sh*n-*l\ aj 1: relating to or indicating direction 
   in space :  1a: suitable for detecting the direction from which radio 
   signals come or  for sending out radio signals in one direction only 1b: 
   operating most effectively in a particular direction  2: relating to 
   direction or guidance esp. of thought or effort