ar·bi·trary
 /ˈɑɚbəˌtreri, Brit ˈɑːbətrəri/  adj  
  1 [more ~; most ~] : not planned or chosen for a particular reason
   An arbitrary number has been assigned to each district.
  : not based on reason or evidence
   I don't know why I chose that one; it was a completely arbitrary decision.
  2 : done without concern for what is fair or right
   Although arbitrary arrests are illegal, they continue to occur in many parts of the country.
  ar·bi·trari·ly /ˌɑɚbəˈtrerəli, Brit ˈɑːbətrərəli/  adv  
   The number was arbitrarily chosen.
  ar·bi·trari·ness /ˈɑɚbəˌtrerinəs, Brit ˈɑːbətrərinəs/  noun   [  noncount  ]