"Original Source" is a term equivalent to the traditional "Primary Source". It has been adopted by the The Board for Certification of Genealogists as the preferred term for genealogical research, and contrasts with their Derivative Source, which is equivalent to the traditional "Secondary Source". The BCG intent is probably to make the distinctions between "primary" and "secondary" sources clear to the lay genealogist. Presumably by describing something as the "Original" source the idea that it is a source contemporary with the events in question is better conveyed, and more accessible to the average genealogist. Their reasoning is given on their web site at BCG. Note that their explanation does not actually address the question that they raise. However, their FAQ does give a good explanation of what they mean by their preferred terms. See also Source Value
From: BCG extracted 15 May 2009:
Question: Why don’t genealogists use the simple terms “primary source” and “secondary source”? In school I was taught to use these to decide whether sources were reliable.
Answer: Determining reliability is not a simple matter. The terms “primary source” and “secondary source” theoretically distinguish reliable sources from potentially unreliable ones. As a point of fact, however, in various fields the terms are used ambiguously by researchers in a number of contradictory ways. Attempting to make a “simple” either-or choice does not enable a researcher to evaluate historical evidence reliably.
Genealogical practice appraises reliability in three ways. We appraise the source (its physical form), the quality of the information within that source, and the type of evidence we can draw from that information. Each of those aspects has two basic qualities. The following provides a brief tutorial:
SOURCES: Sources can be people, artifacts, documents, or publications (printed or digital). They are either
INFORMATION: In using a source, we evaluate separately each ”information statement,” to determine whether it offers
EVIDENCE: Information that is relevant to the problem is considered evidence. It may be one of two basic types: