della Porta, Donatella (2006), Social movements : an introduction, Malden, MA : Blackwell Publishing

Social movements are:

a distinct social process, consisting of the mechanisms through which actors engage in collective action:

• are involved in conflictual relations with clearly identified opponents
• are linked by dense informal networks
• share a distinct collective identity

Mario Diani (1992, 2003, 2004, Diani & Bison 2004)