Adjective necessary has 2 senses
  1. necessary - absolutely essential
    unnecessary, unneeded, excess, extra, redundant, supererogatory, superfluous, supernumerary, surplus, gratuitous, needless, uncalled-for, inessential, reserve, spare
  2. necessary - unavoidably determined by prior circumstances; "the necessary consequences of one's actions"
    evitable, avoidable, avertible, avertable (indirect, via inevitable)
,Noun necessary has 1 sense
  1. necessity, essential, requirement, requisite, necessary - 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"
    --1 is a kind of thing
    --1 has particulars: desideratum; must; need, want