essential

modif.

1. Absolutely necessary

   imperative, required, indispensable, requisite, necessary, vital; see also crucial, necessary 1.

2. Rooted in the basis or essence

   basic, primary, quintessential; see fundamental 1, inherent.

essential, in strict usage, is applicable to that which constitutes the absolute essence or the fundamental nature of a thing and therefore must be present for the thing to exist, function, etc. food is essential to life; an indispensable person or thing cannot be done without if the specified or implied purpose is to be achieved her diagnostic skills proved indispensable to the clinic; requisite is applied to that which is required by the circumstances or for the purpose and generally suggests a requirement that is imposed externally rather than an inherent need the requisite experience for a position; necessary implies a pressing need but does not always connote absolute indispensability