authoritative
adj.adj. having power and control, or showing that you are confident and knowledgeable. People trust an authoritative source because it is reliable.
adj. possessing or exercising authority; commanding respect and confidence through competence or official status. Often used to describe sources, voices, or tones that are definitive and trustworthy.
The doctor gave an authoritative answer to our question.
She wrote the report in an authoritative tone that convinced the board to approve the budget.
The historian's authoritative account of the war remains the standard reference for scholars, despite newer interpretations emerging in recent decades.
From authorit(y) + -ative.
Often modified by 'highly' or 'very'. Commonly collocates with 'source', 'voice', 'tone', or 'figure'.