Most of the tyre looks fine, true and straight but the section photographed shows bulging. The rim itself is true, is this safe to ride on?
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Short answer, no it is not safe.
This looks like the tire bead is not correctly seated (not clinching the clincher) in which case deflating and reseating will likely fix the issue (check the bead...if it's damaged, the tire is done).
The other, less likely, possibility is a weak area on the tire. This is usually accompanied by a scrape or other damage to the sidewall of the tire...can't really tell in this picture, but I suspect you would have mentioned such.
In either case, riding on this can lead to a blowout. As it's a rear tire, this is not quite the horror it is with a front tire, but it still is not a lot of fun.
Be safe, fix or replace.
In general, no -- not safe. Some tire/rim combos don't center up well and can produce a version of the above even when reasonably well seated, but generally the effect isn't as extreme as what I see in the picture.
First inspect the tire for actual bulges, suggesting torn cord in the tire (which means the tire must be replaced). If you don't find those, let all the air out of the tire, wiggle it around until it's evenly centered, then re-inflate. If it looks OK after refilling, ride it around the block once and then recheck, and check again every 10-15 miles for the first 50.
Sometimes, for tires that refuse to center well, it works to inflate the tire to about half pressure and ride it slowly a hundred feet or so, to make the tire seat itself. (Then fully inflate, of course.)
It may be that you have the wrong size tire for the rim.
Do not ride until you have inspected it. I would suggest that you remove the tire and inspect the inside in the area in question. If there are no areas of dryrot or damage you're ok to use it providing the bead is fully seated. Also check to see if a twist or kink in the wire bead may be the cause of the bubble. As long as the bead seats correctly on the rim a little kink won't hurt anything.