If you look up the dictionary definition"
“lacking some property, order, or distinctness of structure previously or usually present.”
Basically it means a surface that lacks the usual properties needed in order to function well as surface in a CAD application. This is due to the fact that the math does not work as expected.
Rhino’s failure to identify degenerate surfaces is a huge disservice to the Rhino users. Many users think that because there is no warning given that the geometry is good when it is not.
One of the many bad things that may happen is as a result is failure when trying to export geometry to other CAD programs.