Web Definition: a diagram that uses circles to represent mathematical or logical sets pictorially inside a rectangle (the universal set); elements that are common to more than one set are represented by intersections of the circles
According to Wikipedia:
Venn diagrams were introduced in 1880 by John Venn (18341923) in a paper entitled “On the Diagrammatic and Mechanical Representation of Propositions and Reasonings” in the “Philosophical Magazine and Journal of Science”, about the different ways to represent propositions by diagrams. The use of these types ofdiagrams in formal logic, according to Ruskey and M. Weston (2005), is “not an easy history to trace, but it is certain that the diagrams that are popularly associated with Venn, in fact, originated much earlier. They are rightly associated with Venn, however, because he comprehensively surveyed and formalized their usage, and was the first to generalize them”.