Obviously there is a bit of contradiction here, which is why this term isn't universal -- if it were "definitive" in the way you describe it, it wouldn't be an estimate.
The PMBOK Guide suggests that there are multiple estimates associated with a project. The final estimate would be prepared when most of the information about the project is known -- the work breakdown structure has been completed, there is a design or approach for the project, and all planning is complete.
This final estimate might be considered the "definitive" estimate. A definitive estimate is one which is carefully put together with due diligence in consideration of the project facts. Like any estimate, it is still only an estimate and may be exceeded or be an over estimate, usually due to project risks becoming issues, whether these risks have been identified or not.