"Heat waves" refers to prolonged periods of unusually hot weather. It means the temperature outside is significantly higher than usual for an extended period of time. Full definition
Rising temperatures on earth — and the increasing frequency of heat waves in particular — cause lower agricultural yields. (sciencedaily.com)
The graph shows the fraction of heat waves for which models projected global warming would be the primary contributor. (climate.gov)
In some cities, he says, fewer people seem to die during heat waves if the average temperature has been higher and they have got used to the heat. (newscientist.com)