A Plus Size Tankini is a swimsuit with a hip length top and either a regular swim suit bottom, swim shorts or a swim skirt - which is sometimes called a skirtini. There is such a great selection available and these can be very comfortable and flattering.
I find they are much more comfortable than a one-piece suit. With my pear shaped body, I love the extra hip coverage from the shorts or skirt. I have a skirtini with a longer skirt (mid-thigh length) and it is so much more flattering than almost any other suit. Of course, it can be a little bothersome when swimming - all that extra fabric - I guess that is the trade off.