Lean principles, originating from Japanese automotive industry, are anticipated to be useful to improve software development processes. Albeit its popularity there is still no generally accepted, clear and detailed definition of what lean software development actually means. This makes it difficult to perform research on the effects of lean software development and determine its usefulness in various contexts. To fill in that research gap this paper analyzes the state of the art based on 20 key Lean concepts derived from nine seminal sources identified in a systematic literature review. The original explanations of the key concepts have been elaborated further and synthesized into a framework for lean software development consisting of a set of goals, recommended activities and practices. The detailed results for the key concept Value are reported. The proposed framework is expected to serve as a basis for further research and for Lean assessment of organizations.