A gravitational mirage is the visual impression caused by a so-called gravitational lens

Gravitational lens

A gravitational lens refers to a distribution of matter between a distant source and an observer, that is capable of bending the light from the source, as it travels towards the observer...

 in space, which may include multiple images, rings, etc. of background light sources. This is analogous to the familiar mirage

Mirage

A mirage is a naturally occurring optical phenomenon in which light rays are bent to produce a displaced image of distant objects or the sky. The word comes to English via the French mirage, from the Latin mirare, meaning "to look at, to wonder at"...

 which can frequently be observed where the air temperature varies strongly with height over the ground (or sea). The rapidly changing refractive index

Refractive index

In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

produces inverted images "floating" in the air.