Emergence, a concept that fi rst appeared in philosophy, has been widely explored in the domain of complex systems and is sometimes considered to be the key ingredient that makes 'complex systems' 'complex'. Our goal in this paper is to give a broad survey of emergence defi nitions, to extract a shared defi nition structure and to discuss some of the remaining issues. We do not know of any comparable surveys about the emergence concept. For this presentation, we start from a broadly applicable approach and fi nish with more specifi c propositions. We fi rst present fi ve selected works with a short analysis of each. We then propose a merged analysis in which we isolate a common structure through all defi nitions but also what we think needs further research. Finally, we briefl y describe some perspectives about the emergence engine idea also referred to as emergent engineering.

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