Scientific name for the union point of a plant stem and root
From Basconsuelo et al 2002:

"The zone of the plant axis where the arrangement of vascular systems changes from root- to stem-structure is often called “transition region”" (see also the refs they cite).

The idea behind it is that the organization of the vascular tissues is radically different in the root in comparison to the stem, such that the switch cannot occur in one point. This transition region (in general case) crosses the air-ground interface.