Internal boundary

of a region $D$ in the Euclidean space $\mathbf{R}^n$

The set $\partial D \setminus \partial(\complement\bar D)$, where $\partial D$ is the boundary of $D$ and $\partial(\complement\bar D)$ is the boundary of the complement of the closed region $\bar D$.

Comments

The internal boundary is also called the inner boundary.