This overview by Michael Quinn Patton and Ricardo A. Millett provides a definition and discussion of how lessons learned can be derived and from what sources.
"The ill-defined, trivial, and over-generalized ways in which the phrase “lessons learned” has come to be used challenges evaluators to distinguish between casual/informal notions of lessons learned and what we call “high quality” lessons learned. High-quality lessons learned contain knowledge that can be applied to future action and can be derived from screening according to specific criteria"
Patton, M. Q. and Millett, R. A., (1998). Lessons Learned, The evaluation exchange, Volume IV, Number 3 & 4. Retrieved from: https://archive.globalfrp.org/evaluation/the-evaluation-exchange/issue-archive/learning-organizations/lessons-learned
'Lessons learned' is referenced in: