Checks – A separation of the wood normally occurring across or through the rings of annual growth and usually as a result of seasoning.
Grain – The fibers in wood and their direction, size, arrangement, appearance or quality.
Knots – A portion of a branch or limb that has become incorporated in a piece of lumber. In lumber, knots are classified as to form, size, quality and occurrence.
Pitch – An accumulation of resinous material.
Pitch streaks – A well-defined accumulation of pitch in the wood cells in a streak.
Shake – A lengthwise separation of the wood which occurs between or through the rings of annual growth.
Skips – Areas on a piece that failed to surface clean.
Slope of grain – The deviation of the line of fibers from a straight line parallel to the sides of the piece.
Split – A separation of the wood through the piece to the opposite surface or to an adjoining surface due to the tearing apart of the wood cells.
Stain – A marked variation from the natural color.
Unsound wood (decay) - A disintegration of the wood substance due to action of wood-destroying fungi, also known as dote or rot.
Wane – Bark or lack of wood from any cause, except eased edges, on the edge or corner of a piece of lumber.
Warp – Any deviation from a true or plane surface, including bow, crook, cup, and twist or any combination thereof.
1All definitions taken from Western Lumber Grading Rules 98