1. final and conclusive; not able to be changed or improved upon
2. most complete and accurate; serving as the best example of its kind

Examples

• The Supreme Court’s ruling was definitive and could not be appealed.
• Scientists are still waiting for definitive proof of the theory.
• This is considered the definitive biography of Abraham Lincoln.
• The test results gave us a definitive answer about the diagnosis.
• Her definitive performance of the role earned critical acclaim.
• The committee reached a definitive decision after hours of debate.
• This dictionary is the definitive guide to American English.