The rehaut ("flange" in English) is typically a ring that separates the dial from the crystal inside a watch.
The French term "rehaut" refers to a flange, and can include many elements of the construction of a watch. However, in horology, this term generally refers to the inner ring placed beneath the crystal and above the dial of a watch, creating space for the hands and other elements of the watch face. Design-wise, the rehaut is usually a complement to the bezel, which sits outside the crystal.
Some watches lack a rehaut entirely, with the crystal and dial fitting into a slot directly in the case. But since many dials have imperfections or mechanical elements around the edge, a rehaut is usually preferrable stylistically. Other rehauts are engraved to be the chapter ring of the watch, and these terms are often incorrectly used interchangeably. Technically, any "inner bezel" is a rehaut, though decorated or movable rehauts (sometimes found on dive watches) are usually called bezels instead.