A heat wave is a period with high temperatures
In The Netherlands we are going out of a heat wave if there are at least 5 summer days (25 degrees or higher) consecutively, there must also be another 3 tropical (30 degrees or higher). In general, this is based on the temperature in de Bilt (if one is based on a different place or region, it is a regional heat wave and not a rural one). Heat waves have been maintained since 1901 by the KNMI. The heat wave of 2018 was just not the longest heat wave since the measurements (15 days).