You have a class hierarchy:
D extends C which extends B which extends A, for example.
A would be the top class, but how about D? I am looking for a short and concise way of describing the position of D within the hierarchy
Is it the most specific class? the deepest class in the hierarchy? the last subclass? final class?
The class might or might not be final (java) or sealed (c#). what matters is that it is the last one of the hierarchy at a particular moment. If a newer class (E) appears that extends D then the proposed term would apply to it. Also, there might be more than one class that fit this description (both D and E extends C), the term should apply to both classes.