 Karoubi envelope

The Karoubi envelope freely splits idempotents. Its objects are pairs $(A,e)$ and a morphism $(A,e)\to (B,f)$ is a map $u:A\to B$ satisfying $u=fue$.

The Karoubi envelope, also known as the Karoubi construction or Karoubi's sheaf, is a concept in the field of homotopy theory and algebraic topology, particularly associated with the study of motivic homotopy theory and stable homotopy categories.