So you know what you're reading when you're looking at directions, or so you can talk to your arborist and communicate exactly what you mean, here's a collection of terms you need to know when pruning a tree.
Branch Axil: the angle formed where a branch joins another branch or stem of a woody plant.
Branch Bark Ridge: a ridge of bark that forms in a branch crotch and partially around the stem, resulting from the growth of the stem and branch tissues against one another.
Branch Collar: a "shoulder" or bulge formed at the base of a branch by the annual production of overlapping layers of branch and stem tissues.
Crown Cleaning: removal of dying, diseased, dead and broken branches in the tree crown.
Crown Raising: a method of pruning to provide clearance for pedestrians, vehicles, buildings, lines of sight, and vistas, by removing lower branches.
Crown Reduction Pruning: a method of pruning used to reduce the height of a tree. Branches are cut back to laterals that are at least 1/3 the diameter of the limb being removed.
Crown Thinning: a method of pruning to increase light penetration and air movement through the crown of a tree by selective removal of branches.
Callus: see woundwood.
Decurrent: a major tree form resulting from weak apical control. Trees with this form have several to many lateral branches that compete with the central stem for dominance, resulting in a spherical or globose crown. Most hardwood trees have decurrent forms.
Detail Pruning: pruning ornamental trees for appearance. Pruning enhances natural shape.
Epicormic Sprout: a shoot that arises from latent or adventitious buds; also know as water sprouts that occur on stems, and branches and suckers, produced from the base of trees. In older wood, epicormic shoots often result from severe defoliation or radical pruning.
Excurrent: a major tree form resulting from strong apical control. Trees with this form have a strong central stem and pyramidal shape. Lateral branches rarely compete for dominance. Most conifers and a few hardwoods, such as sweetgum and tuliptree, have excurrent forms.
Flush Cuts: pruning cuts that originate inside the branch bark ridge or the branch collar, causing unnecessary injury to stem tissues.
Fruit Tree Pruning: crown thinning with an emphasis on dead wood removal and water sprout thinning; pruning to encourage fruit production, air circulation and to strengthen fruit bearing branches.
Included Bark: bark enclosed between branches with narrow angles of attachment, forming a wedge between the branches.
Pollarding: the annual removal of all of the previous year's growth, resulting in a flush of slender shoots and branches each spring.
Stub Cuts: pruning cuts made too far outside the branch bark ridge or branch collar, that leave branch tissue attached to the stem.
Tipping: a poor maintenance practice used to control the size of tree crowns; involves the cutting of branches at right angles leaving long stubs.
Topping: a poor maintenance practice often used to control the size of trees; involves the indiscriminate cutting of branches and stems at right angles, leaving long stubs. Synonyms include rounding-over, heading-back, dehorning, capping and hat-racking. Topping is often improperly referred to as pollarding.
Topiary: the pruning and training of a plant into a desired geometric or animal shape.
Woundwood: lignified, differentiated tissues produced on woody plants as a response to wounding (also known as callus tissue).
Courtesy of the US Dept. of Agriculture.