Lessons learned or lessons learnt are experiences distilled from a project that should be actively taken into account in future projects.
There are several definitions of the concept. The one used by the National Aeronautics and Space Administration, European Space Agency and Japan Aerospace Exploration Agency sounds as follows: “A lesson learned is knowledge or understanding gained by experience. The experience may be positive, as in a successful test or mission, or negative, as in a mishap or failure...A lesson must be significant in that it has a real or assumed impact on operations; valid in that is factually and technically correct; and applicable in that it identifies a specific design, process, or decision that reduces or eliminates the potential for failures and mishaps, or reinforces a positive result.” 
The Development Assistance Committee of the Organisation for Economic Co-operation and Development defines lessons learned as “Generalizations based on evaluation experiences with projects, programs, or policies that abstract from the specific circumstances to broader situations. Frequently, lessons highlight strengths or weaknesses in preparation, design, and implementation that affect performance, outcome, and impact.” 
In the practice of the United Nations the concept has been made explicit in the name of their Working Group on Lessons Learned of the Peacebuilding Commission. In the military field, conducting a Lessons learned analysis requires a leader-led after-actions debriefing. These debriefings require the leader to extend the lessons-learned orientation of the standard after-action review. He uses the event reconstruction approach or has the individuals present their own roles and perceptions of the event, whichever best fits the situation and time available.
- [Secchi, P. (Ed.) (1999). Proceedings of Alerts and Lessons Learned: An Effective way to prevent failures and problems (Technical Report WPP-167). Noordwijk, The Netherlands: ESTEC]
- [OECD – DAC (2002) Glossary of Key Terms in Evaluation and Results Based Management. Evaluation and Aid Effectiveness No 6. http://www.oecd.org/dataoecd/29/21/2754804.pdf]
- Department of the Army (2009). Field Manual No. 6-22.5. Combat and Operational Stress Control Manual for Leaders and Soldiers. Department of the Army Headquarters, Washington, DC, 18 March 2009. p. 50