tree species
One of the three main elements by which a tree is defined. The others are tree size and tree form. A tree species is a perennial species having secondary thickening (ie. true wood) and characteristically attains tree form and tree size when mature. There is no clear distinction between a tree species and other species.
This definition is abridged from A – Z of tree terms: A companion to British arboriculture. Order the book, read the preface.