Definition of various grafting terms.
Grafting is a method of asexual plant propagation widely used in agriculture and horticulture where the tissues of one plant are encouraged to fuse with those of another. It is most commonly used for the propagation of trees and shrubs grown commercially.
In most cases, one plant is selected for its roots, and this is called the stock or rootstock.
Scion: The tree part being grafted onto the rootstock is usually called the scion.
Rootstock: A rootstock is a plant, and sometimes just the stump, which already has an established, healthy root system, used for grafting a cutting or budding from another plant.
Graft Union: The graft union refers to the point at which the scion and understock of a graft meet, or the visible scar that appears at this interface when the texture or color of the bark doesn't quite match.
The scion is the plant which has the properties desired by the propagator, and the rootstock is the working part which interacts with the soil to nourish the new plant.
Source:
Wikipedia - Rootstock