Definition - What does Graft Union mean?
A graft union is the anatomy which is formed when grafting a rootstock to a scion. This can be in the case of grape vines, but it is a universal term used for any type of grafting of trees, tomatoes, etc. A successful graft union will be noted when the plant grafted is emitting sap and calluses over at the site of the graft.
WineFrog explains Graft Union
A graft union is also used to describe a specific place by which a graft should be formed in order to increase the chances of having a successful graft union. A graft union is not always successful, and sometimes, the scion (a branch of another genus in the same species) will be rejected. This graft union creates a hybrid. In the case of a successful graft, the graft union will begin to form sap created a scab to heal and protect the vine for proper growth.
During the Great French Wine Blight, American rootstocks were grafted onto French scions, causing a blight due to the phylloxera aphid being introduced into the French crops from the phylloxera-resistant American rootstocks. The site of where these two vines were joined is referred to as the graft union.