A heat wave is an extended interval of abnormally hot and usually humid weather, usually lasting from a few days to over a week.
The Weather Channel uses the following criteria for a heat wave:
A minimum of ten states with 90 degree plus temperatures and the temperatures must be at least five degrees above normal in parts of that area for at least two days or more.
The intensity of the heat necessary to be called a heat wave varies with the location.
A period of hot, humid weather in Chicago, Detroit, or New York could be considered normal in Atlanta, New Orleans, or Houston. This is because sporadic hot weather where it is less common is more taxing on the population than prolonged heat and humidity where it is typical.
Since the stress of heat is dependent on geographic area and human tolerances of the climate, heat-related problems are more likely to occur earlier in the summer, like in June, than later in the season, like in August. Northern and urban areas are not easily adaptive to occasional heat waves.