Two adjacent VoXels are said to be 6-connected when they share one common face. The name comes from the fact that a 3D voxel has six facets.
A voxel also has eight corners (vertices) and twelve edges. Other kinds of connectivity are therefore possible: you could consider two voxels to be connected (part of the same object) if they share one edge, for example. Still, at present the Huygens Object Analyzer groups voxels together only when they are face-connected, which is quite convenient to provide robustness to the analysis of noisy data or small scattered objects.