The skeleton and its properties
The notion skeleton was introduced by H. Blum as a result of the Medial Axis Transform (MAT) or Symmetry Axis Transform (SAT). The MAT determines the closest boundary point(s) for each point is in an object. An inner point belongs to the skeleton if it has at least two closest boundary points.
A very illustrative definition of the skeleton is given by the prairie-fire analogy: the boundary of an object is set on fire and the skeleton is the loci where the fire fronts meet and quench each other.
The third approach provides a formal definition: the skeleton is the locus of the centers of all maximal inscribed hyper-spheres (i.e., discs and balls in 2D and 3D, respectively). (An inscribed hyper-sphere is maximal if it is not covered by any other inscribed hyper-sphere.)