I think it’s a common English term (i.e. “shallow water” to describe a small lake-like thing where you can go without swimming), but non-native speakers may not know it (I don’t remember if I knew it before learning Python, for example).

What about this:

 All slice operations return a new list containing the requested elements.  This
-means that the following slice returns a shallow copy of the list *a*::
+means that the following slice returns a shallow copy (see the documentation of
+the :mod:`copy` module for a definition) of the list *a*::

(BTW, you can use syntax like Doc/tutorial/introduction.rst:487 to have links generated; see http://docs.python.org/devguide/triaging#generating-special-links-in-a-comment —also linked from the “Comment” label in the form, but it isn’t obvious that it’s a link).