vattendiagram
Vattendiagram is a term sometimes used colloquially to refer to a Venn diagram, a graphical representation used in set theory to illustrate all possible logical relations between a finite collection of different sets. A Venn diagram uses closed curves, typically circles, to represent sets. The elements of each set are placed within the corresponding circle. The relationships between sets are shown by the way the circles overlap or do not overlap.
The overlapping areas, or intersections, of the circles represent the elements that are common to the sets
While "Vattendiagram" is not a standard mathematical term, it is likely a phonetic or typographical variation