What is a thunderstorm downburst?

From the National Weather Service

A downburst is a localized area of damaging winds caused by air rapidly flowing down and out of a thunderstorm. To create a downburst at the ground, the downward (downdraft) speeds in the thunderstorm must be unusually high, and this downward flowing air must penetrate close to the ground. These conditions can be met when the boundary layer air is relatively dry, and when there is plenty of falling precipitation. It is not necessary for the thunderstorm to produce hail or tornadoes to produce a downburst. Damage from downbursts can be so severe that it is mistaken for tornado damage. When examined, however, the damage pattern from a downburst will be divergent, indicating the winds were flowing outward, rather than in a circular pattern as in the case of a tornado. Downburst damage can cover hundreds of square miles, or be limited to a single field (the smallest ones are called "microbursts").