Knot

A knot is the base of a branch, embedded in the trunk and preserved in the wood once the tree is milled into lumber -- visible as a distinct, usually darker, circular or oval patch where the branch's own grain crosses the surrounding trunk grain at an angle. A tight knot is still fully fused to the surrounding wood and structurally sound; a loose knot has partially or fully separated from the surrounding wood (common once a branch has died and the tree has grown around the stub) and can fall out entirely, leaving a hole.

Structurally, a knot is close to always a weak point -- the grain disruption it causes concentrates stress and makes that spot more prone to splitting or breaking than clear, straight-grained wood nearby, which is exactly why structural grading standards for framing lumber specifically limit knot size and location. Aesthetically, though, knots are frequently a deliberate, sought-after character feature rather than a defect to avoid -- knotty pine paneling and rustic furniture both build their whole look around leaving knots visible rather than cutting around them.