necessary

[1]	necessity , essential , requirement , requisite

adj.

[1]	indispensable , essential , required , needed , compulsory , requisite , vital , demanded , imperative , obligatory , needful , of the essence , important , of the utmost importance , top-priority , high-priority , urgent , exigent , compelling , life-and-death/life-or-death
[2]	inevitable , unavoidable , inescapable , ineluctable
[3]	sure , certain , predetermined , predestined , fated , inexorable , resulting , resultant