A burl is a naturally occurring growth on a tree, often appearing as a rounded, knotted protrusion on the trunk, branches, or roots. Unlike the smoother contours of typical tree growth, a burl is formed when the tree undergoes stress from injury, virus, fungus, or insect infestation. This disruption causes the wood grain to grow in a chaotic, interlocked manner rather than in the straight lines seen in regular wood.