Since then, the basic idea of collective action and forward progress has taken such a dizzying variety of forms — liberal and conservative, grass-roots and top-down, “poor people’s movements” and “rich people’s movements” — that scholars have a hard time settling on a single meaning for what a social movement is and does. “The term has multiple definitions,” write the editors of the Encyclopedia of American Social Movements, “but perhaps the simplest one is the best: the coming together of large numbers of people to pursue a goal that they believe will improve society.” Movements are supposed to be different from other modes of citizen engagement, less beholden to elections and hierarchies, more fluid and open to transformative ideas. Often their goals are explicitly political, but the word is also used more casually these days, describing almost any group effort aimed at changing cultures, lifestyles or habits. There are craft-beer and slow-fashion and furniture-free movements. The ad firm Ogilvy & Mather champions the “wellness movement,” with “the new space it creates for brands and marketers,” while employers of all stripes proclaim their membership in the workplace “diversity movement.”

Even the most traditional social movements tend to change over time, further complicating the problem of definition. One commonly identified shift came in the 1960s, when “new social movements” began to experiment with street protest and consciousness-raising and nonviolent direct action in new ways. As initially understood by sociologists, these social movements came almost exclusively from the left. But the postwar era also proved to be fertile ground for movements on the right, ranging from the student activists of Young Americans for Freedom to “massive resistance” in the white South. Leaders of the self-identified “conservative movement” spent at least part of that decade policing their own borders and defining their ideological tenets, and by the 1980s, these “movement conservatives” were running the Republican Party.

On the right, many activists focused their energies on transforming or taking over existing institutions, including national parties and local school boards. On the left, movements sometimes rejected such institutional goals, pushing for sweeping changes in hearts and minds, perhaps with legislation or court decisions to follow. In his 2011 book, “American Dreamers,” the historian Michael Kazin identifies a persistent historical divide in outcomes as well, arguing that the left has been far better at “helping to transform the moral culture” than at presenting “a serious challenge to those who held power in either the government or the economy.” This distinction has basically held true for more recent movements. In the wake of the financial crisis, the Tea Party took control of a large share of the Republican Party, while Occupy Wall Street proved most effective at promoting a popular language to criticize the “1 percent.”

One thing they have in common, though, is that each came and went relatively quickly. Less than a decade from their origins, the Tea Party and Occupy already seem like anachronisms. Micah White, one of Occupy’s founders, argues in his 2016 book, “The End of Protest,” that today’s movements need a thorough reinvention if they hope to withstand the slings and arrows of opposition over the long term. From another perspective, though, the rapid turnover of today’s movements may actually be a sign of their success. “Occupy begat We Are the 99% begat Fast Food Forward begat $15 Now begat the Bernie Sanders campaign,” wrote Eric Liu in last year’s “You’re More Powerful Than You Think” — while the Tea Party “harnessed a radical anti-establishment spirit that seized and then consumed the Republican Party, fueled Donald Trump’s election, unleashed a new populism and created a ‘none-of-the-above’ opening for libertarians.” In this vision, we’re already living in a golden age of citizen activism, when even high school students have the tools to organize a nationwide protest in five weeks flat.

Whether that protest, or any other, will finally be deemed a “movement” no doubt depends on what it ultimately accomplishes. One open secret of social activism is that nobody can ever really predict when, where, how or why any given issue will change from a lost cause to a cause célèbre. As my Yale colleague and gay rights pioneer Evan Wolfson often reminds students, ambitious goals have usually seemed “impossible” until they were achieved, at which point they suddenly became “inevitable,” a matter of simple justice and common sense. The movement is what happens in between./•/