Onion model
The onion model is a graph-based diagram template for describing an expanding or extending relationship between several concepts. The name is a metaphor of the layered shells that become visible when you cut open an onion. The outer layers in the model typically add size or complexity incrementally to the central layers.
The onion diagram can be presented as an Euler diagram or a stacked Venn diagram. For the record, a stacked Venn contains a series of sets A1...Ak where each set An is a strict subset of An+1 (and by recursion, of all Am where m > n). However, as the onion is often used as a metaphor, it doesn't necessarily make sense to interpret the contents of an onion diagram literally or mathematically.
This diagram format is supported e.g. by Microsoft PowerPoint's SmartArt wizard under the name of "stacked Venn".
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