What is a three-body civilization?

Three-body civilization is an advanced alien civilization. A fictional alien civilization in Liu's science fiction novel "Three-body", known as the First Earth Branch of Three-body, is an advanced alien civilization with strong scientific, technological and organizational capabilities, which once posed a great threat to the earth civilization. Three-body is a series of long science fiction novels written by Liu. It consists of three works: Three-body, Three-body Dark Forest and Three-body Death and Eternal Life. This work tells about the information exchange, the life-and-death struggle and the rise and fall of the two civilizations in the universe. The story originated from the three-body civilization, which is an alien civilization located in three solar systems.