Taper roll has been used to describe slightly warped shafts. However, in recent years, it's become much more liberal & is being used to describe shafts that are a little more than slightly warped. Truth is, I have never seen an absolutely perfect straight shaft. IMO, it doesn't exist. But there is a point where 99%+ people agree that it's acceptable straight. That would be a shaft that doesn't noticeably wobble when rolled with or without the butt. And every shaft that has a pro, compound, or parabolic taper all will show light under it. The amount of variance you see in that light as the shaft rolls is an indication of how straight the shaft actually is. Slight variance is what folks call "taper roll". No that's not exactly correct terminology but everybody knows it means slightly bowed.
Taper roll is an uneven cut. When tapering a shaft, there's always slight (very slight) variance in density from one side to the other. That's no big deal except when the cutter is getting dull. Then it begins pushing a tiny bit on the harder side of the shaft, which causes the softer side to reflex in a weencie bit deeper. It's still a clean & smooth cut but an ultra slightly uneven cut. And it can be seen when rolled on the table as a slight variance in light. Shaft is still straight, just slightly uneven cut. Still plays & feels fine. Once the bit gets dull enough, it begins causing vibrations & leaving strange cut patterns, and that's when the builder knows it's time to sharpen or change the blade. But for a few shafts before the builder knows, there possibly will be a true "taper roll".
Another thing that causes roll off is the joint work. Once shaft is to size, the joint work is done. Then it's cut to size to mate with the butt. If it's only .001" off center at the joint then it'll be compounded enough 15" down the shaft to see when rolled. For some reference, a human head hair is .004"-.006" thick, so .001" is tiny and is a common benchmark for cue machining. It's within the accepted accuracy range. But even still, it'll show slight roll. Maybe not enough to see for one guy but enough to see for another guy. Then you must consider the finish and if it's perfectly symmetrical & even. If it's slightly thicker one side to the other, it'll show a slight roll. Same with joint facing (while screwed together only).
Point being, there's a lot of variables that a builder must fight to get a perfectly straight shaft. It's virtually impossible. But there is an acceptable "straight" and anybody who considers themselves a cue maker can easily meet that range. Brand new off the lathe a shaft may have a true "taper roll" but you'd have to be super nit to see it & complain about it. But somewhere I suspect a builder tried explaining this all to a super nit & labeled it "taper roll" and from there it became synonymous with minor warpage. In actuality it's nothing even remotely close to warpage but is caused due to a number of very minor machining flaws. Hope this helps.
