absolutely necessary; indispensable: Discipline is essential in an army.
pertaining to or constituting the essence of a thing.
noting or containing an essence of a plant, drug, etc.
being such by its very nature or in the highest sense; natural; spontaneous: essential happiness.
Mathematics. (of a singularity of a function of a complex variable) noting that the Laurent series at the point has an infinite number of terms with negative powers. (of a discontinuity) noting that the function is discontinuous and has no limit at the point. Compare removable (def 2).