Definition: Anything artificial on the course, with exceptions for any objects that define out of bounds, any construction that the local committee defines as an integral part of the course, or any immovable artificial object that is out of bounds.
There are movable obstructions and immovable obstructions, and I bet you can figure out the difference between them. How a player deals with an obstruction depends on whether the obstruction is movable or immovable.
In the rulebook, obstructions are covered in Rule 24.
