Other forms: knotholes
A knothole is a naturally-occurring open space in a wooden board. You wouldn't want to use boards full of knotholes to build a boat, but they're fine for a backyard tree house.
Many types of wood form round, raised imperfections called knots, which mark the spot where a branch used to be before it died and fell off the tree. Many paneled rooms are lined with "knotty pine," a light-colored wood marked with many of these knots. When a knot falls out of a cut length of wood, it leaves a knothole behind.