(1) Authoritative and final. (2) Specifying perfectly or precisely.
Example:
The team's brilliant research provided a definitive description of the virus and its strange mutation patterns.
Something definitive is complete and final. A definitive example is the perfect example. A definitive biography contains everything we'll ever need to know about someone. Ella Fitzgerald's 1950s recordings of American popular songs have even been called definitive, though no one has ever wanted them to be the last.