Definitions of Technical Terms

Arborist: An arborist/arboriculturist, is a professional in the practice of arboriculture.

An informal term is tree surgeon.

Arboriculture: The cultivation, management, and study of individual trees, shrubs and vines, and other perennial woody plants.
Bracing: A term used to desribe the installation of cables, ropes or/and belts to reduce the probability of failure of one or more parts of the tree structure due to weakened elements under excessive movement.
Canopy: The top layer of a tree, sometimes called the crown.
Cavity: A void within the solid structure of the tree, normally associated with decay or deterioration of the woody tissues. They can be dry or they can normally hold water, if it does contain water then it needs to be drained.
Coppicing: The cutting down of a tree within 300mm (12 inches) off the ground at regular intervals, traditionally applied to certain species such as Hazel and Sweet Chestnut to provide stakes etc.
Crown: The foliage bearing section of the tree formed by its branches and not including any clear stem/trunk.
Crown Cleaning: The removal of any epicormic growth, crossing branches or dead wood. Leaving the tree looking clearer and removes potential hazards from heavy branches.
Crown Lifting: The removal of lower branches to lift the height of the crown. This is appropriate for roadside locations or for allowing more light into adjacent properties.
Crown Thinning: The removal of weak or crossing branches to improve the structure of the crown and let in more light. This can give the tree the appearance of being smaller and less imposing. 
Deadwood: Non-living branches or stems due to natural ageing or external influences. Deadwood does provide essential habitats and its management should aim to leave as much as possible, shortening or removing only those that pose a risk. The durability and retention of deadwood will vary by tree species.
Decline: When a tree exhibs signs of a lack of vitality such as reduced leaf size, colour or density.
Dieback: Tips of branches exhibit no signs of life due to age or external influences. These declines may progress, stabilise or reverse as the tree adapts to its new situation.
Dormant: The inactive condition of a tree, usually during the coldest months of the year when there is little or no growth and leaves of deciduous trees have been shed.
Drop Crotching: This is the shortening of branches by pruning off the end back to a lateral branch which is at least 1/3 of diameter of the removed branch.
Epicormic: “An epicormic shoot is a shoot growing from an epicormic bud which lies underneath the bark of a trunk, stem or branch of a plant” (Taken from Wikipedia)
Felling: To cause a tree to fall by knocking down a tree.
Formative Pruning: Minor pruning during the early years of a tree’s growth to establish the desired form and/or to correct deffects or weaknesses that may affect structure in later life.
Fungi/Fruiting Bodies: A member of the plant kingdom that may colonise living or dead tissue of a tree or form beneficial relationships with the roots. The fruiting body is the spore bearing, reproductive structure of that fungus.

Removal of the fruiting body will not prevent further colonisation.

Heave: Ground movement at base of tree, normally appears in dangerous trees in high windy when the tree looks like it could uproot.
Lopping: The removal of side sections of a tree to pull away from buildings, services or other structures.
Painting or Sealing: This is the action of covering pruning cuts or other wounds with a paint, often bitumen based. Research has shown that this is not beneficial for trees and may even harm. On no account should timber treatment be used as they are definately harmful to living cells.
Prune: To cut off or remove dead or living parts or branches to improve shape or growth.
Reduction: The removal of the outer crown to produce a smaller sized canopy. Preferably no more than 30% of the tree should be removed at any one time unless safety is an issue, although this varies from species to species.
Retrenchment Pruning: A form of reduction intended to encourage development of lower shoots and emulate the natural process of tree ageing.
Root Pruning: The pruning back of roots (similar to pruning back the branches). This has the ability to affect the stability of a tree so it is advisable to seek professional advice prior to attempting root pruning.
Stump Removal: The removal of the stumps left in the ground after the tree has been felled. The stump is the bottom part of the trunk with the roots attached underground.
Suckering: Where new shoots grow from the truck and/or roots.
Tree Preservation Orders (TPOs) A legal order that protects the tree. Means that before any work can be done on the tree, from general pruning to full felling/removal of tree permission from the local council must be received. Any tree can be placed under a TPO despite its age, size or species.