Authoritative describes something that comes from a trusted expert or reliable source and can be counted on as accurate. An authoritative book on dinosaurs is written by paleontologists who've spent years studying fossils, not by someone guessing what dinosaurs looked like. An authoritative voice on climate science belongs to researchers who've carefully measured temperatures and analyzed data over decades.

The word indicates real knowledge, expertise, or official power behind what's being said. A teacher speaking authoritatively about mathematics knows the material inside and out and demonstrates that mastery. When a historian writes an authoritative biography, other scholars respect it because the research is thorough and trustworthy.

You'll also hear authoritative describe someone's manner or tone: commanding and confident in a way that makes others listen. A camp counselor might use an authoritative voice when giving safety instructions, speaking firmly so everyone knows to pay attention and stay safe.