I am trying to find the official (or a good enough) reason that the free store is commonly referred to as the heap.

Except for the fact that it grows from the end of the data segment, I can't really think of a good reason, especially since it has very little to do with the heap data structure. 

Note: Quite a few people mentioned that it's just a whole bunch of things that are kind of unorganized. But to me the term heap physically means a bunch of things that are physically dependent on one another. You pull one out from underneath, everything else collapses on it, etc. In other words, to me heap sounds loosely organized (e.g., latest things are on top). This is not exactly how a heap actually works on most computers, though if you put stuff towards the beginning of the heap and then grew it I guess it could work.