absolutely required and not to be used up or sacrificed
absolutely necessary; vitally necessary; "essential tools and materials"; "funds essential to the completion of the project"; "an indispensable worker"
being or relating to or containing the essence of a plant etc; "essential oil"
a deliberate act of omission; "with the exception of the children, everyone was told the news"
an instance that does not conform to a rule or generalization; "all her children were brilliant; the only exception was her last child"; "an exception tests the rule"
grounds for adverse criticism; "his authority is beyond exception"