The polar bear can breed with the brown bear, and the offspring would probably be fertile.
But it would never happen in the wild. The ranges of polar and brown bears do not overlap, and even if two met, they would see each other as competitors, not mates.
Such interbreeding has occurred in captivity, but not in more than two decades. In the early 1980s, an Asiatic black bear and a spectacled bear interbred, but the male offspring was sterile. Modern-day zoo management frowns on these things.
The polar bear has been formally called both Urdu,’ maritimus and Thalarctos maritimus. Urdu,’ maritimus, meaning sea bear, was originated in 1774 by Commander C. J. Phipps, a British naval officer and author of A Voyage Towards the North Pole.
The more recent designation Thalarctos maritirnus combines the Greek thalasso, or sea, and arctos, or bear of the north.
Arctic itself means “Country of the Great Bear,” but the name comes from a bear in the night sky, the Great Bear constellation, or Arktos.