A burl (British "burr") is an outgrowth on a tree in which the grain has grown in a deformed manner (commonly found in the form of a rounded outgrowth on a tree trunk or branch), which yields a very peculiar and highly figured wood. Most burls grow beneath the ground, attached to the roots as though a type of malignancy that is not discovered until the tree dies, or falls over. The burl sometimes looks like bulbous partial round pieces of small to large and heavy pieces often connected by a system of rope-like roots. Almost all burl wood is covered by bark even though it is often underground. Insect infestation and certain types of mold infestation are the most common causes of this condition. Burls are often misunderstood. As it is the product of a cambium it may not be compared to any phenomenon in animals or humans. A burl is only a burl if it is filled with small knots from dormant buds. Some say that burls are near knots but cannot have knots themselves, and are the result of a tree that undergoes some form of stress, be it environmental or introduced by man

Burr wood is an uncommon and rare type of wood that is prized for its beauty, but because the grain is misshapen and not straight it is very hard to work in a lathe or with hand tools, which also adds to its expense. Some burls are more highly prized than others, including the Burls originating in rural areas in and around central Massachusetts, recently discovered in northeast Connecticut and as far south as Philadelphia, but some burls seem to get an explosion of sorts which causes the grain to grow erratically and it is these burls that the artist prizes over all other types.

In some tree species, burls can grow to great size. Some of the largest occur in redwoods (Sequoia sempervirens); when moisture is present, these burls can grow new redwood trees. Many redwood trees have them, but there are two things that hold back the harvest of redwood burls. One is that the removal of the burl can cause the death of the tree. The other is that the burls of redwoods can be of tremendous size, and at abnormal depths, so the removal of these burls require heavy equipment, which can be expensive and at times unable to get to the area of the tree due to the terrain.

There are a number of famous burls (each from a particular species): The world's largest and second-largest burls can be found in Port McNeill, British Columbia. One of the largest burls known to man was found around 1984 in the small town of Tamworth, Australia. It stands 6.4 ft tall with an odd growth resembling a trombone.

The famous Birdseye maple superficially resembles the wood of a burl but is something else entirely.