Adventitious
Describing shoots which develop other than from terminal or axillary buds; also roots which form other than through primary development.

Arboriculturist
A person who, through relevant education, training and experience, has recognized qualifications and expertise in the field of trees.

Arisings
Any parts of a tree, including stem, roots, limbs or brushwood, bark, other woody material and foliage derived from the tree during tree work operations.

Bolling
Pollard heads collectively, or sometimes the entire framework of a pollarded tree.

Branch collar
An anatomical feature of a branch union.

Canker
An area of dead cambium and overlying tissues killed by a pathogen.

Cavity
A hole in a woody part of a tree caused by decay or damage. Cavities may hold water (wet cavity) or be dry.

Coppicing
Cutting trees down to ground level (usually within 75 mm of the ground) with the intention of encouraging regrowth of multiple shoots.

Crotch
A forked region formed by the junction of a branch and the trunk, or by two branches.

Crown
The main foliage-bearing part of a tree.

Crown lifting
The removal of lower branches to achieve a stated vertical clearance above ground level or other surface.

Crown reduction
An operation that results in an overall reduction in the height of a tree and usually also its spread by means of a general shortening of twigs and/or branches.

Crown thinning
The removal of a proportion of small, live branches from throughout the crown to ensure an even density of foliage around a well-spaced and balanced branch structure.

Epicormic
The shoots or roots which are initiated on mature woody stems.

Heart rot
The decay in the central core of wood of a trunk, stem or branch.

Heartwood
The dead inner layers of wood in the trunk or branch of a tree, sharply delineated, often by colour, from sap wood.

Knuckle
A swelling that forms at a pollard point especially after repeated cutting.

Senescence
The third phase of a tree's life, after youth and maturity, when the leaf area of the crown tends to diminish. Veteran trees are generally senescent.

Stump
The base of a trunk that remains after felling.

Tree Surgeon
A contractor skilled in performing aerial operations in trees.

Topping
The removal of most or all of the crown of a mature maiden tree or the pruning of a pollarded tree below the knuckles.

Wetwood
A condition in living trees where the wood develops an abnormally high water content and water soaked appearance.

Sources:
BS 3998 Recommendations for tree work