From AMS Glossary
(Also called scattergram.) A plot representing corresponding observed values of two variables x and y as points in Cartesian coordinates.
If the two variables are functionally related, the points will be bunched, but if they are not functionally related, the points will be scattered uniformly over the plane. Scatter diagrams are used to explore the influence of one variable upon another, strong relationships being revealed as a concentration around a definite curve.