A heat wave is a period of unusually warm or very hot weather, and may be accompanied by high humidity. "Heat wave" is not an exact term: A heat wave happens when it is warmer than normal for multiple days. For example, temperatures that are normal for a warmer climate might be called a "heat wave" in a place that is usually cooler. A "heat wave" could mean warm weather, (like "several days as hot as summer, but in winter"), or temperatures so high they make history, (such as, "the hottest summer in a hundred years").
