In other words, it's a loaded term, interpreted in myriad ways depending on the source (sort of like the story of the 5 blind men and the elephant: each felt a different part of the elephant's body, and so described the elephant solely based on the part they were feeling --- yeah, hold off on the dirty jokes, thankyou!
).
From my perspective, "aiki" is the process of connecting to and controlling someone's center -- and thus, his body -- using IP/IS. It is a product of an internal-strength conditioned body; you can't have aiki without that.
But if you talk to someone who trains in contemporary aikido, they would give you a completely different interpretation because the internal component of aikido, as utilized by founder Morihei Ueshiba, has largely been lost. That is, unless the aikido person you ask has been training, lately, with one of the folks who are teaching IP/IS to motivated aikido folks in hopes that it will be incorporated back into the art.