Originaly from the Latin language a consensus has two meanings. The first one is the theory and practice where a group of people debate with and listen to each each other until there is an agreement with everyone concurring. This agreement is also called a consensus which is the second definition.
Forming a consensus is a common way to lead to agreements from a group where there is no leader. Because anarchic groups, some community based organisations, collaborations and collectives have no leader, consensus is usually the way they reach agreement.
Actions taken before achieving a consensus from related stackholders is called unilateralism.