Lumber is wood that has been cut and prepared for building. When a tree is cut down and sawed into flat boards, beams, and planks, it becomes lumber. The two-by-fours used to frame a house, the plywood sheets that become floors, and the boards that form a deck are all lumber.

Before a tree becomes lumber, it must be processed at a sawmill, where huge spinning blades slice the logs into standard sizes. A carpenter building a treehouse needs lumber, not whole trees. A furniture maker might buy special hardwood lumber like oak or maple, while construction workers use softer woods like pine.

As a verb, lumber can also mean moving in a heavy, clumsy way. When a bear lumbers through the forest, it moves with a slow, powerful gait, its weight making each step deliberate. Unlike a deer that bounds gracefully, a lumbering creature moves with impressive but awkward momentum.