basic, called for, cardinal, central, compulsory, critical, crucial, dictated, essential, exigent, fundamental, high-priority, imperative, important, imposed, integral, irreplaceable, key, main, major, mandatory, necessarius, necessary, necessitated, needed, obligatory, of importance, pivotal, pressing, primary, required, requisite, significant, unavoidable, urgent, vital, wanted
associated concepts: indispensable evidence, indispensable parties
cardinal (basic), central (essential), compulsory, essential (required), exigent, fundamental, grave (important), imperative, important (urgent), instrumental, integral, mandatory, material (important), necessary (required), necessity, obligatory, requisite, urgent, vital
Burton's Legal Thesaurus. William C. Burton. 2006
The Essential Law Dictionary. — Sphinx Publishing, An imprint of Sourcebooks, Inc. Amy Hackney Blackwell. 2008.