Three sets of three lines such that each line is incident with two from both other sets.
Steiner Set
See also
Solomon's Seal Lines, Steiner Points, Steiner Triple SystemExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Steiner Set." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/SteinerSet.html