Term: Inner Boundary
IB 
Definition: A worst-case boundary generated by the collective effects of the smallest feature of size (MMC for an internal feature of size, LMC for an external feature of size) and the applicable geometric tolerance.
Citation: ASME Y14.5 : Dimensioning and Tolerancing
See Also:
Feature of Size, Maximum Material Condition, Least Material Condition, Geometric Element, Tolerance