Essential

Crucial

Synonyms: Crucial, key, indispensable, required, requisite, all-important, of the utmost importance, of the essence, life-and-death, mandatory, obligatory, urgent, burning, paramount, high-priority, consequential, necessary, vital, needed, called for, important, vitally important, of great consequence, critical, imperative, compulsory, compelling, pressing, acute, pre-eminent, and significant.

Basic

Synonyms: Basic, fundamental, intrinsic, characteristic, rudimentary, principal, chief, elemental, pivotal, key, salient, vital, indispensable, ingrained, inherent, quintessential, underlying, innate, primary, cardinal, elementary, central, critical, focal, staple, necessary, and foundational.

Necessity

Synonyms: Necessity, prerequisite, requirement, condition, specification, qualification, sine qua non, necessary/essential item, requisite, need, precondition, stipulation, desideratum, and must.

Fundamentals

Synonyms: Fundamentals, rudiments, first principles, preliminaries, essence, core, nub, meat, bedrock, hard facts, realities, nitty-gritty, nuts and bolts, basics, principles, foundations, groundwork, basis, kernel, marrow, crux, facts, practicalities, sine qua non, brass tacks, and ABC.

Example Sentences

Water is essential for life.

Safety is essential at work.

Vitamins are essential for health.

It's essential to stay focused.