au·thor·i·ta·tive / əˈ[unvoicedth]ôriˌtātiv; əˈ[unvoicedth]är-/ • adj. 1. able to be trusted as being accurate or true; reliable: clear, authoritative information. ∎ (of a text) considered to be the best of its kind and unlikely to be improved upon: the authoritative study of mollusks.2. commanding and self-confident; likely to be respected and obeyed: she had an authoritative air. ∎ proceeding from an official source and requiring compliance or obedience: authoritative directives.DERIVATIVES: au·thor·i·ta·tive·ly adv.au·thor·i·ta·tive·ness n.