soil that contains all (i.e.. container grown) or a portion (i.e.. B&B) of the roots that are moved with a plant when it is planted or transplanted.
Merriam-Webster Online Dictionary
root ball (noun) the compact mass of roots and soil formed by a plant especially in a container
root ball (Wikipedia)
Root ball of a Phoenix canariensis palm tree.
A root ball is the main mass of roots at the base of a plant such as a shrub or tree. It is of particular significance in horticulture when plants are repotted or planted out in the ground. The quality and preparation of the root ball will determine how well the plant will survive this transplantation and then flourish in its new situation.