Roundabouts actually exist in some states. In Massachusetts they are called rotaries. Here in California, and in other states that have them, they are called traffic circles as per the definition above.

The last time my mom was in the UK (visiting us when we lived there) she said that the very small roundabouts with no grass in the middle, just pavement, were new to her (we lived in London when I was a toddler) and she called them "virtual roundabouts."