What is the definition of topping a tree?
The Morton Arboretum Tree-Care Handbook calls topping “indiscriminately sawing off large branches.” (1994)
According to the Washington State Department of Ecology’s Vegetation Management: A Guide for Puget Sound Bluff Property Owners (May 1993, p.23), the practice of topping usually refers to cutting the upper portion of the main leader (trunk) in conifers and to the removal of all branches at a particular height in deciduous trees. Topping is not advised.
Plant Amnesty has loads of information about topping – including why it should not be done – at the following link: 5 Reasons Not to Top.