Robert Mueller

“In evaluating whether evidence about collective action of multiple individuals constituted a crime, we applied the framework of conspiracy law, not the concept of "collusion." In so doing, the Office recognized that the word "collud[e]" was used in communications with the Acting Attorney General confirming certain aspects of the investigation's scope and that the term has frequently been invoked in public reporting about the investigation. But collusion is not a specific offense or theory of liability found in the United States Code, nor is it a term of art in federal criminal law. For those reasons, the Office's focus in analyzing questions of joint criminal liability was on conspiracy as defined in federal law.”
― Robert Mueller, The Mueller Report: The Comprehensive Findings of the Special Counsel