Bandwagon, usually heard as "jumping on the bandwagon," is the act of adopting a behavior, belief, or affiliation because others already have rather than on the merits.
The phrase is literal in origin. Nineteenth-century American political campaigns hired circus bandwagons to draw a crowd, and local figures would climb aboard to be seen with a popular candidate. By the 1890s "jump on the bandwagon" had become shorthand for attaching yourself to a movement already gathering momentum, and it has carried a faintly pejorative edge ever since — applied to sports fans who appear at the playoffs, investors who arrive late to a mania, and adopters who discover a thing once it stops being a risk.
As a mechanism it is the plain-language name for the 📝Bandwagon Effect, a 📝Cognitive Bias in which the likelihood of adopting something rises with the number of people who already have. The distinction is worth keeping: bandwagon names the observable behavior, the effect names the causal pattern underneath it, and 📝social proof is the lever used to trigger it on purpose.