<p><strong>Adjective:</strong></p><ul><li>absolutely necessary; indispensable: Discipline is essential in an army.</li><li>pertaining to or constituting the essence of a thing.</li><li>(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.</li><li>(of a discontinuity) noting that the function is discontinuous and has no limit at the point.</li></ul><p><strong>Noun:</strong></p><ul><li>a basic, indispensable, or necessary element; chief point: Concentrate on essentials rather than details.</li></ul>