Crosses often appear with a circle or arc, and these can be called Halo Crosses.
Synonyms for Halo include Nimbus, Aureole, Glory and Gloriole.
The word halo comes from the Greek halos, which means the ring of light shown around the Sun. The word was borrowed for the name of the light, real or imaginary, appearing around a holy person. Artists have used this to represent the spiritual light from holy people "The righteous shall shine forth as the Halo in the kingdom of their Father." (Matthew 13:43)
A circle or arc is much easier to draw or carve than an image of a holy person, therefore just the halo is sometimes shown on its own to represent a holy person.
Halos are not restricted to Christianity; other religions such as Buddhism, Hinduism and Jainism use halos in iconography.
In a Christian context, a Halo Cross is a generic term for many crosses with such shapes and a few examples are shown on the left.