material witness(n = noun.person) - a witness whose testimony is both relevant to the matter at issue and required in order to resolve the matter;
is a kind of witness

N  evidence, facts, premises, data, praecognita, grounds, indication, criterion, testimony, testification, expert testimony, attestation, deposition, examination, admission, authority, warrant, credential, diploma, voucher, certificate, doquet, docket, testamur, record, document, pi ce justificative, deed, warranty, signature, seal, exhibit, material evidence, objective evidence, witness, indicator, hostile witness, eyewitness, earwitness, material witness, state's evidence, deponent, sponsor, cojuror, oral evidence, documentary evidence, hearsay evidence, external evidence, extrinsic evidence, internal evidence, intrinsic evidence, circumstantial evidence, cumulative evidence, ex parte evidence, presumptive evidence, collateral evidence, constructive evidence, proof, evidence in chief, secondary evidence, confirmation, corroboration, support, ratification, authentication, compurgation, wager of law, comprobation, citation, reference, legal research, literature search (experiment), showing, indicative, indicatory, deducible, grounded on, founded on, based on, corroborative, confirmatory, by inference, according to, witness, a fortiori, still more, still less, raison de plus, in corroboration, of, valeat quantum, under seal, under one's hand and seal, dictum de dicto, mise en evidence.

abrasive material, absorbent material, adhesive material, adsorbent material, adverse witness, animal material, bear witness, bear witness against, bearing false witness, bedding material, building material, character witness, coloring material, colouring material, combustible material, composite material, contrast material, covering material, create from raw material, expert witness, false witness, fencing material, hostile witness, insulating material, jehovah's witness, lay witness, material, material body, material breach, material possession, material resource, packing material, paving material, photographic material, piece of material, plant material, promotional material, radioactive material, raw material, reading material, roofing material, sealing material, semiconducting material, sorbent material, upholstery material, waste material, waste of material, witness, witness box, witness stand, written material