  Defeaturing consists in simplifying geometrical models by removing the
geometrical features that are considered not relevant for a given simulation.
Feature removal and simplification of computer-aided design models enables
faster simulations for engineering analysis problems, and simplifies the
meshing problem that is otherwise often unfeasible. The effects of defeaturing
on the analysis are then neglected and, as of today, there are basically very
few strategies to quantitatively evaluate such an impact. Understanding well
the effects of this process is an important step for automatic integration of
design and analysis. We formalize the process of defeaturing by understanding
its effect on the solution of Poisson equation defined on the geometrical model
of interest containing a single feature, with Neumann boundary conditions on
the feature itself. We derive an a posteriori estimator of the energy error
between the solutions of the exact and the defeatured geometries in
$\mathbb{R}^n$, $n\in\{2,3\}$, that is simple, reliable and efficient up to
oscillations. The dependence of the estimator upon the size of the features is
explicit.
