A layer of wood formed in a plant during a single period of growth. Growth rings are visible as concentric circles of varying width when a tree is cut crosswise. They represent layers of cells produced by vascular cambium. ◇ Most growth rings reflect a full year's growth and are called annual rings. But abrupt changes in the environment, especially in the availability of water, can cause a plant to produce more than one growth ring in a year. See more at dendrochronology.
A similar layer in a part of an animal marking a period of growth, such as an annulus in a fish scale.