Terms: I.S., I.P., Neijin, Fajin, Aiki, etc.
Just for general discussion:
"Internal Strength" is a loose translation of the term "Nei Jin" (roughly: "Internal Trained Force Skill"). The point is that "neijin" is a very common term, so "Internal Strength" would be the closest translation that is generally suitable.
"Internal Power" I don't know anything about, but it seems to be a term that is a square peg in a round hole.
"Fa Jin" is also sometimes loosely intermixed with "Fa Li". "Fa Jin" is literally "attack jin"; "Fa Li" is literally "attack strength".... the implication in both is a *whole body shaking issuance of strength*. "Li" by itself is just the word for strength and often implies normal muscular strength.
"AiKi" implies a 'mixing of ki-strength' or a 'mixing of ki'. All strength and movement comes from ki in the body (which is interrelated with fascial strength), so a weight lifter can have 'strong ki', but he may not be able to manipulate that ki in the optimal way with his hara/middle/dantien. So if the aforementioned weight-lifter attacks someone who is skilled in 'internal strenght', there could be a mixing of forces which in turn defeats the weight-lifter. Hence, "Ai Ki".