A Mexican wave
A wave effect formed when crowds in stadia rise up and down from their seats in succession.
Given that name when the phenomenon came to a mass audience during the football World Cup held in Mexico in 1986. It had originated somewhat earlier, probably in US colleges. This piece from The New York Times, October 1984, pre-dates the football tournament:
"This undulating human wave ... apparently became popular at University of Washington football games a few years ago."
That 'apparently' indicates that the New York Times had no proof. Nevertheless, there are strong representations from supporters of that view, who claim 'The Wave' was first performed on October 31, 1981 at an American football game at the University of Washington's Husky Stadium.
The action itself, whatever those involved at the time called it, certainly pre-dates 1981. In the Fred MacMurray comedy film Son of Flubber, 1963, the crowd at an American football game can clearly be seen to enact what we would now call a 'Mexican Wave'.