The Met Office defines a heatwave as "an extended period of hot weather relative to the expected conditions of the area at that time of year, which may be accompanied by high humidity". In other words, it's hotter for a longer period of time than you would expect, for where you are.
This is a companion discussion topic for the original article at https://patient.info/news-and-features/how-to-cope-in-the-outdoors-with-a-heatwave