A monokini is a one piece bikini. A one piece swimsuit usually has a lot of coverage, whereas a monokini has minimal coverage like a bikini.
Monokinis like this yellow one are ultra sexy, have a thong back and are often worn by exotic dancers. They are also fun for romantic private jacuzzi / pool use.
Many monokinis offer more coverage, but are still sexy, like this black O-ring style. From the back it looks like a regular 2 piece full back bikini, but it is really a one piece, monokini.
I like these because you can get a great tan (unlike in a one piece) and never have to worry about losing your bottoms while body surfing in the ocean!