As far as fusion is concerned, you can look at it as a reaction/expansion in the late 60s where jazz started to absorb other musics around it. Acid jazz does that too, but as Elson said really is simpler in structure and harmony than many of the fusion albums (and unfortunately many of the players are less talented IMHO). And there is too much out there to absorb these days. Hence it gets confusing....think "is portishead acid jazz".
One of the thing I learned in my jazz class last year at school was that there was a definite alteration in jazz during the second world war...before the war there was swing & big band jazz, the "revolutionary dance music" of the time, the one that people's parents were warning them about etc. Post war jazz became head music, music you would go and sit to, music that was infinitely more intricate harmonically but had unfortunately moved away from 'the groove'.
I argued in a paper for that class that acid jazz is best considered not by the influences that it absorbs because the categorization of music is IMHO fairly meaningless, only serving to help a listener identify albums based on broad generalizations. Instead, acid jazz, whatever the hell it is, is best thought of as an attempt to regain the spirit of dancing that was lost from jazz in the 30s, I guess much like funk (but some funk music is pretty much spiced up blues and there is a bit more harmonic exploration in what I think of when I think of acid jazz).
Enough.
Jay B