A golf ball is considered out of bounds when the entire ball lies outside the boundary edge of the course. This boundary edge is typically marked with white stakes or lines. If any part of the ball touches the course-side edge of the boundary line, it is considered in bounds.

Here's a more detailed explanation:
Boundary Edge:
The boundary edge is the defining line separating the course from out of bounds.
Out of Bounds Definition:
A ball is out of bounds when it is entirely outside this boundary edge.
Impact of Stakes and Lines:
White stakes or lines are commonly used to mark the boundary, but other methods like fences or walls can also define the out of bounds area.
Touching the Edge:
If any part of the ball is touching the course-side edge of the boundary line or stake, it's considered in bounds.
Penalty:
If a ball is out of bounds, the player must take a one-stroke penalty and replay the shot from the original spot, according to the rules of stroke and distance (Rule 14.6).