A minimal bikini, the bottom of which consists of one, two, or three narrow strings. The one and three string versions could be considered to provide a very minimal amount of coverage, but the two string version leaves the wearer wide open. If a matching top is worn, it also consists of a few strings that provide very little coverage. At the-Bikini.com these are called "PeekABoo" bikinis.
Photos of Invisible Bikinis
Vanessa in her three string "frame thong"