Venn diagram

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A diagram using circles to represent sets, with the position and overlap of the circles indicating the relationships between the sets.

Wiktionary

1. n. set theory A diagram representing some sets by contours of closed shapes, such as circles or ellipses (and sometimes also the universal set as a rectangle enclosing all of these shapes), and indicating the relationships between the sets: by overlapping the shapes to show that the corresponding sets have a non-empty intersection, and by possibly (but not necessarily) enclosing all of the sets (which are proper subsets of the universal set) within a universal set (represented typically by a rectangle); such that the total number of simply connected regions is , where n is the number of depicted sets which are proper subsets of the universal set.

WordNet 3.0

1. n. 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