What is a tankini?

When you consider the swimsuit styles available, a few things are clear. One-piece suits are back in a big way, and bikinis will always be popular. But there's still one swim style that you may not be so sure about -- the tankini. You may not see many tankinis making their way down the runway during Miami Swim Week, but you're sure to see plenty of them at the beach or pool. With such enduring appeal, what does the tankini have to offer?