Euler diagram
a pictorial image of how entities or situations intersect or overlap. Whereas a Venn diagram depicts all areas of intersection, a Euler diagram does not necessarily include all areas. For example, a researcher might create a Venn diagram in which a large circle represents a population, a circle inside that represents a sample or subset of the population, and overlapping circles inside the previous one indicate additional subsets. If the image were to leave out some of the inner circles, such as the ones representing different kinds of random samples, it would be considered a Euler diagram. [Leonhard Euler (1707–1783), Swiss-born Russian mathematician]