(burr) an abnormal swelling of a tree trunk characterized by swirling wood grain and meristematic tissue. Wood with these structures is prized for woodworking. Contrast with gall.
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A burl (American English) or bur or burr (UK English) is a tree growth in which the grain has grown in a deformed manner. It is commonly found in the form of a rounded outgrowth on a tree trunk or branch that is filled with small knots from dormant buds.