Sylvester–Gallai configuration
In geometry, a Sylvester–Gallai configuration consists of a finite subset of the points of a projective space with the property that the line through any two of the points in the subset also passes through at least one other point of the subset. Instead of defining Sylvester–Gallai configurations as subsets of the points of a projective space, they may be defined as abstract incidence structures of points and lines, satisfying the properties that, for every pair of points, the structure includes exactly one line containing the pair and that every line contains at least three points.
Source: Wikipedia — Sylvester–Gallai configuration (CC BY-SA 4.0)