au·thor·i·ta·tive [? tháwr? tàytiv] adjective 1. reliable: convincing, reliable, backed by evidence, and showing deep knowledge 2. backed by authority: backed by an established and accepted authority 3. showing authority: showing confidence in or the expectation of being obeyed -au·thor·i·ta·tive·ly, adverb -au·thor·i·ta·tive·ness, noun