vouch

from The American Heritage® Dictionary of the English Language, 5th Edition.

• intransitive verb To give personal assurances or a guarantee.
• intransitive verb To constitute supporting evidence; give substantiation.
• intransitive verb To substantiate by supplying evidence; prove.
• intransitive verb Law To summon (someone) as a witness to give warranty of title.
• intransitive verb To refer to (an authority, for example) in support or corroboration; cite.
• intransitive verb To assert; declare.
• noun A declaration of opinion; an assertion.

from The Century Dictionary.

• noun Approving or supporting warrant; confirmation; attestation.
• To call to witness.
• To declare; assert; affirm; attest; avouch.
• To warrant; be surety for; answer for; make good; confirm.
• To support; back; second; follow up.
• In law: To produce vouchers for, in support of a charge in account.
• In old English law, to call or summon into court to warrant and defend, or to make good a warranty of title.
• Synonyms To asseverate, aver, protest.
• To bear witness; give testimony or attestation: more specifically, in old English law, to call in some one to make good his alleged warranty of title; be surety or guaranty.

from the GNU version of the Collaborative International Dictionary of English.

• intransitive verb To bear witness; to give testimony or full attestation.
• intransitive verb To assert; to aver; to declare.
• noun obsolete Warrant; attestation.
• transitive verb obsolete To call; to summon.
• transitive verb To call upon to witness; to obtest.
• transitive verb To warrant; to maintain by affirmations; to attest; to affirm; to avouch.
• transitive verb To back; to support; to confirm; to establish.
• transitive verb (Law) To call into court to warrant and defend, or to make good a warranty of title.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb To take responsibility for; to express confidence in; to witness; to obtest.
• verb To warrant; to maintain by affirmations; to attest; to affirm; to avouch.
• verb To back; to support; to confirm.
• verb To call into court to warrant and defend, or to make good a warranty of title.
• verb To bear witness; to give testimony or full attestation.
• verb To assert; to aver; to declare.
• noun Warrant; attestation.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb give personal assurance; guarantee
• verb give supporting evidence
• verb give surety or assume responsibility
• verb summon (a vouchee) into court to warrant or defend a title