BRANCH - A secondary shoot or stem arising from one of the main axes (i.e., trunk or leader) of a tree.
BRANCH BARK RIDGE - The raised area of bark in the branch crotch that marks where the branch wood and trunk wood meet.
BRANCH COLLAR - Trunk tissue that forms around the base of a branch between the main stem and the branch. As a branch decreases in vigor or begins to die, the branch collar becomes more pronounced.
CALLUS - Tissue formed by the cambium layer around and over a wound.
CANOPY - Upper portion of the tree consisting of limbs, branches, and leaves.
CAMBIUM - Dividing layer of plant cell that forms sapwood (xylem) to the inside and bark (phloem) to the outside.
CLEAN CUTS - Cuts made using a sharp tool, with no nicks or tears on the branch collar or the trunk.
CLOSURE - Refers to callus growth covering of a cut or other tree wound.
CROWN - Technically, the juncture of the trunk above the roots, but in common usage, it refers to the foliage comprising the uppermost branch structure.
CROWN ELEVATION - A regional term synonymous with underclearance. See UNDERCLEARANCE.
CUT - The exposed wood area that remains after the branch has been removed.
CUTTING BACK - Pruning designed to reduce the crown of a tree or individual branch. Sometimes referred to as heading back, drop crotch pruning, natural pruning, lateral pruning, or directional pruning. It is distinctly different from "topping" (see definition) in that an effort is made to keep the symmetry of the tree on the sides as well as the top.
DECAY - The degradation of plant tissue, including wood, by pathogens such as fungus organisms. Wood decay can reduce the structural integrity of a tree or its individual limbs.
DORMANT - A state of inactivity, or no growth. Deciduous trees are considered to be dormant from the time the leaves fall until ne w foliage begins to appear.
DROP CROTCH PRUNING - The specific cutting back of a branch or leader to a lateral branch at least one-third to one-half the diameter of the cut being made.
GIRDLING ROOTS - Roots located above or below ground level whose circular growth around the base of the trunk or over individual roots applies pressure to the bark area, thereby choking or restricting the flow of sap.
LATERAL - A side branch or twig.
LEADER - A dominant upright stem, usually the main trunk.
LUTING - The removal of lower branches for under-clearance.
LIMB - Same as branch.
LINE CLEARANCE - Pruning for the safe operation and maintenance of uninterrupted electric service.
PARENT STEM - The main trunk system of the tree: also, the dominant leader of a major limb.
PHOTOSYNTHESIS - The process by which green plants manufacture (carbohydrates) in cells containing chlorophyll, utilizing sunlight for energy.
POLLARDING - Trees cut back to essentially the same point seasonally, resulting in multiple sprouts above the cuts.
PRECUT or PRECUTTING - The removal of the branch far enough beyond the finished cut so as to prevent splitting into the parent stem.
PRUNING - The removal of plant parts, dead or alive, in a careful and systematic manner so as to not damage other parts of the plant.
SAP FLOW - The course assumed by sap in its movement through a tree.
SAPWOOD - A wood layer of variable thickness found immediately inside the cambium, comprised of water-conducting vessels and living plant cells. Also know as xylem.
SCARS or INJURIES - Natural or man-made lesions of the bark in which wood is exposed.
SUCKER - A vigorous shoot arising at or below the graft union. WATERSPROUT.
SUNBURN - Bark injury cause by extreme heat from the sun.
SUNSCALD - Bark splitting or injury caused by temperature extremes or sudden temperature fluctuation.
THINNING - The removal of branches where they arise in order to let in light, reduce wind resistance, remove unwanted branches, or to retain a tree's natural shape.
TOPIARY - Trees sheared or pruned carefully in a formal shape.
TOPPING - The severe reduction of branches without consideration of the specifications for cutting back. (This is generally considered to be an undesirable practice.)
TRACING - Careful removal of the loose or damaged bark along the edges of a wound to encourage closure.
TRIMMING - See PRUNING.
UNDER-CLEARANCE - The removal of lower tree limbs to allow clearance beneath the tree crown. The same as CROWN ELEVATION.
WATER SPROUT - A vigorous shoot arising from the above-ground portion of the tree or above the graft union. See SUCKER.
WOUND - The opening that is created any time the tree's protective bark covering is penetrated, cut, or removed, injuring or destroying living tissue. Pruning a live branch creates a wound, even when the cut is properly made.

REFERENCES
1. Harris, R. W. 1983. Arboriculture: Care of Trees, Shrubs and Wines in the Landscape. Englewood Cliffs, New Jersey, Prentice-Hall.
2. National Arborist Association Standards For Pruning of Shade Trees, Guying of Shade Trees, Fertilizing Shade and Ornamental Trees, and Lightning Protection Installation for Shade Trees. P.O. Box 1094, Amherst, NH 03031-1094.
3. Shigo, A. L. 1986. A New Tree Biology. Durham, NH, Shigo and Trees
4. Shigo, A. L. 1986. Tree Pruning: A Worldwide Photo Guide. Durham, NH, Shigo and Trees.