Collective effervescence is Emile Durkheim's concept, from Elementary Forms of Religious Life: it is the energy formed by a gathering of people in a crowd or mob that has gathered at a sporting event, a carnival, or something more spontaneous, a riot. This energy, the affective transmission that produces a hightened sense of what's at stake in having agency in the present, produces a different rationality, a different real, a virtually sacred sense of something happening in the realm of collective being. At those moments where the historical present seems to shift, conventions of what's destructive and what's worldmaking become unstable, and everything seems more symbolic--both more enigmatic and more authentic of a collective spirit.
The conventional image of revolution often points to this kind of kinetic tableau, where individuality finds collective agency and the institutions follow, cha cha cha. This is one reason why there's so much fear of popular and populist being in the streets: that it won't be just for show but intends to be a transformative event that takes over the present durably.