A follow-up on "dialects"

In a previous post http://jessicabruin.blogspot.com/2013/04/some-housekeeping-on-names.html, I asked the question, “when does a language become a different language?”  One of my mentors provided a simple answer, "when the language varieties become mutually unintelligible."  This mentor was comfortable using the term ‘dialect’ when talking about language varieties that are mutually intelligible, such as British English and Standard American.  Both are dialects of English.  According to Britton (2003), http://jessicabruin.blogspot.com/2013/04/bibliography.html Isthmus Zapotec is one of six or seven distinct and mutually unintelligible Zapotecan languages.  Britton also addresses the dialect issue, “Classification is further complicated when one considers the degree of splintering that has occurred on a dialectal level within the major branches—Isthmus Zapotec is varied not only between villages in the isthmus, but several variations may occur within a single city, such as Juchitán” (pp. ix-x).  http://jessicabruin.blogspot.com/2013/04/language-maps.html I hope this helps clear up some of the confusion.