A lengthwise grain separation between or through the growth rings of lumber is called?

The term that defines a lengthwise grain separation occurring between or through the growth rings of lumber is "shake." Shake results from natural defects in the wood caused by environmental stresses during its growth or drying process. This defect occurs along the grain and can compromise the structural integrity of the lumber.

Knots refer to the circular remnants of branches within the wood but do not specifically relate to grain separation. Splitting is a more generalized term that can refer to any type of separation in the wood, not limited to growth rings, which makes it less precise in this context. Checking, on the other hand, typically describes small cracks that may develop on the surface of the wood rather than lengthwise separations through the growth rings.

In summary, "shake" is the accurate term for the specified phenomenon because it directly addresses the type of grain separation that occurs in this manner.