Social scientists use the term diffusion to refer to the spread or flow of an innovation across actors in a social system. When we speak of diffusion of social movements, we mean that some element of a social movement (e.g. tactic, frame, ideology, protest, repertoire, campaign) is spreading across some set of actors (e.g. organizations, networks, groups, people, communities, states) in a social system either through direct or indirect networks of communication.