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A polar-ring galaxy is a galaxy with a circle around it of stars and gas. Those that clearly show the pattern have a ring on a plane at odds with the galaxy, polar indicating the case where the planes of the ring and galaxy are at right angles. The ring is thought to be due to a previous interaction with another galaxy. One possibility is a galaxy merging with the center of another, leaving a surrounding ring. Another possibility is that the ring is the remnant of a tidal arm, stripped from a passing galaxy.
Polar-ring galaxies look unusual and exotic in images, but similar rings are being discovered around the Milky Way in the form of stellar streams, some of which are positioned to make the Milky Way a polar-ring galaxy if they were more dense and prominent.