Well, I agree with Cady in that jin does not appear to be as universally nor as explicitly labeled in Japanese arts in the same manner. Where I would perhaps quibble with Cady would be in the details in which jin is being expressed (in this sense) as aiki as I think is steps too far into the application, cause and effect without perhaps enough consideration of all the pieces working together to allow for said cause and effect.
Budd, I see jin more as the raw material to be exploited by aiki. "Peng" is the core, "main" jin. In "making aiki," there are things that you are doing which are separate, or an addition, to the processes being done to make houkei/peng (thanks Chris!
). There are myriad variations and manipulations to keep shifting the balance of In and Yo. For example, different degrees of engagement in the meimon and tandan, and the use of the kwas (gotta find the Japanese term for these) to create the power of opposing forces via the ground, for winding force. Aiki itself includes sensitivity toward what your own body is doing, and the ability to feel the opponent's center to control his body in any chosen direction and a variety of chosen effects. It is the exploitation and manipulation of the power you generate.
But starting at Square One, I believe that describing houkei (peng) and aiki-age are the simplest way to define IP and Aiki to someone with no prior physical exposure.
P.S. How about the term for "kwa," Chris...?