Social movements are at the heart of politics right now. Drawing loosely on work by Charles Tilly, Doug McAdam, Sidney Tarrow, Frencesca Polletta, Jennifer Earl, and others, I would define a social movement using five criteria (listed below). Note that this is a value-neutral definition. It could fit a fascist movement as well as a progressive one, and it doesn’t imply that social movements are preferable to other phenomena, such as political parties. It is merely intended to categorize a set of phenomena so that we can study them.

1. A social movement consists of many autonomous groups and individuals

Movements are polycentric rather than hierarchical or centralized. A single organization can adopt the feel and spirit of a movement by empowering separate groups internally, but it is only an actual movement if those groups are autonomous and collaborate voluntarily.

2. It persists over numerous episodes and campaigns

The marches over the Edmund Pettus Bridge were episodes. The struggle for the Voting Rights Act was a campaign. The Civil Rights Movement consisted of thousands of episodes and several campaigns.

3. It makes demands on holders of power

The power-holders may be governments, firms, media entities, universities, or any other institutions. If people make no demands on any institutions, I wouldn’t define them as participants in a social movement. They might form an important grouping, such as a spiritual revival or a self-help network, but it would not be a social movement.

Just as important, a social movement makes demands on holders of power rather than trying to supplant them. A political party seeks offices. A revolutionary cadre strives to overthrow and replace the state. An established labor union negotiates contracts with a firm. None of these meet the definition of a social movement, which stands apart from the institution and makes demands on it. I acknowledge that a broad-based social movement can encompass elements that act like parties, revolutionary cadres, or collective bargainers. The Civil Rights Movement encompassed the Mississippi Freedom Democratic Party (MFDP), the Black Panther Party, and the Brotherhood of Sleeping Car Porters; but it was a movement because it wasn’t defined by any of those groups.

4. It supports its demands by displaying WUNC

Tilly observes that social movements don’t just express demands; they back them up with four assets that command attention: worthiness, unity, numbers, and commitment. For example, the women’s marches after Trump’s Inauguration demonstrated that millions of people cared (numbers), that they were willing to travel to Washington or stand outside in the streets of their hometowns (commitment), that they were unified (pink pussycat hats, marching on the same day), and that they were worthy. Worthiness can mean respectability: for instance, “We are middle-class women (with some men) who are pillars of our communities, not the kinds of people who usually make trouble.” But worthiness need not mean respectability. Sometimes, highly oppressed groups claim the worthiness that comes from suffering.

WUNC serves as a standard for evaluating the credibility of would-be movements. For example, does holding a sign on the Boston Common demonstrate enough commitment?

5. It imposes limits on itself

Social movements often adopt and attempt to enforce limits on themselves: “We will not run for office or endorse any parties.” “We will negotiate with companies but not form our own businesses.” “We denounce terrorism.” “We call for a new nation but will not commit treason against the existing state.”

No particular form of self-limitation is definitive of social movements. For instance, there are violent and even terroristic social movements; thus nonviolence is not a condition. But it is highly characteristic–perhaps even definitive–of social movements that they choose some limitations that they enforce on their own members. I would propose a functional explanation of this tendency. Coordinating the behavior of many autonomous groups and individuals is extraordinarily difficult. Movements are easily destroyed by internal disagreements or by escalation to a point where they lose their support. Successful movements avoid escalation by choosing limits and economize on the topics that are open to discussion. For instance, if the movement eschews electoral politics, then it needn’t decide who should run for which office. If it renounces violence, it needn’t debate which targets to hit with which weapons.

Thus a social movement can say: “We are [a set of groups] who want [demands] from [institutions], who demonstrate WUNC [in particular ways] and who eschew [certain tactics or ends.]”