Definition:

A "bunker" is a golf course hazard that is a hole or depression in the ground filled in with sand (or a similar material). Bunkers vary greatly in size and shape and depth. They are most commonly found serving as greenside hazards, but also often show up in fairways and alongside fairways.

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.

Same goes for so-called "waste bunkers," which are not technically bunkers because they are not treated as hazards under the rules.

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."

Some specific types of bunkers include:
Pot bunker (a k a pothole)
Cross bunker
Church pews bunker
There is not a separate section of the rules devoted only to bunkers, but the do's and don'ts of playing from bunkers are addressed in Rule 13 (Ball Played as it Lies).

A stroke played out of a bunker is called a "bunker shot."

Also Known As: Trap, sand trap, sand bunker. "Trap" is a vernacular term; only "bunker" is used in the Rules of Golf.

Examples: "My ball landed in the bunker in front of the seventh green."

"I had to blast the ball out of the bunker at No. 12."