In Gambas, a virtual class is a class containing one or more abstract methods (which Gambas calls "virtual methods"), i.e. methods whose interface has been defined but whose implementation has not been. "A virtual class is an hidden pseudo-class that you cannot explicitly manipulate." 
Gambas virtual classes are always named beginning with a "." (e.g. ".ListViewItem") except when being invoked as the ancestor of another class (i.e. using the "inherits" keyword).