necessary in WordNet English dictionary

1. anything indispensable
   "food and shelter are necessities of life"; "the essentials of the good life"; "allow farmers to buy their requirements under favorable conditions"; "a place where the requisites of water fuel and fodder can be obtained"
   necessity  essential  requirement  requisite  necessary 
   

1. absolutely essential
   necessary 
   
2. unavoidably determined by prior circumstances
   "the necessary consequences of one's actions"
   necessary