Because the comet that amateur astronomer Gennady Borisov discovered in August is actually the second interstellar object ever discovered that is entering the solar system, the International Astronomical Union (IAU) has decided to dub it 2I/Borisov, honoring its discoverer as is traditional with comets but indicating its interstellar nature in the name.
The orbit is now sufficiently well known, and the object is unambiguously interstellar in origin; it has received its final designation as the second interstellar object, 2I. In this case, the IAU has decided to follow the tradition of naming cometary objects after their discoverers, so the object has been named 2I/Borisov.
As my regular readers know, I am not a fan of the IAU’s effort to claim the right to name every object in the universe. In this case it has at least made the proper decision.