In the vernacular, one might hear reference to a "grass bunker," a hollowed-out area or depression in which, rather than sand, there is simply more (often deeper) grass. However, a "grass bunker" is not technically a bunker, because it is not a hazard under the rules. It's simply akin to rough.
The official definition of "bunker" from the Rules of Golf is this:
"A 'bunker' is a hazard consisting of a prepared area of ground, often a hollow, from which turf or soil has been removed and replaced with sand or the like.
"Grass-covered ground bordering or within a bunker, including a stacked turf face (whether grass-covered or earthen), is not part of the bunker. A wall or lip of the bunker not covered with grass is part of the bunker.
"The margin of a bunker extends vertically downward, but not upward. A ball is in a bunker when it lies in or any part of it touches the bunker."
