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)

A root ball is the mass of roots and growing media at the base of a plant such as trees, shrubs, and other perennials and annual plants. The appearance and structure of the root ball will be largely dependent on the method of growing used in the production of the plant. The root ball of a container plant will be different than that of the field-harvested “ball and burlap” tree. The root ball is of particular significance in horticulture when plants are being planted or require repotting as, the quality, size, and preparation of the root ball will heavily determine how well the plant will survive being transplanted and re-establish in its new location.

Root ball of a Phoenix canariensis palm tree.
The photo above displays a large rootball ready for transportation in Austria.
The photo above displays a root bound plant that was grown in a container.
The photo above displays two "ball and bur-lapped" (B&B) trees ready for transportation or planting.
The photo above displays a rootball being excavated.
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