essential

[1]	necessity , requirement , requisite , necessary

adj.

[1]	indispensable , necessary , requisite , required , important , imperative , vital , material , quintessential
[2]	fundamental , indispensable , basic , intrinsic , elemental , elementary , principal , primary , key , main , leading , chief
[3]	all-important , all important , crucial , of the essence
[4]	substantive