According to the Stranger Things aftershow Beyond Stranger Things, the Upside Down isn't actually referred to by that name in the scripts.
For two seasons, the alternate dimension that invades reality and is the home of monsters like the Demogorgon and the Mind Flayer has been referred to as the Upside Down. But while the characters on the show exclusively use that name, the series' aftershow revealed that the script actually calls it The Nether. In fact, during the first season, the cast and crew referred to it as such whenever they weren't on camera.
By now, the name has taken hold among viewers, so it's unlikely many would ever refer to it as The Other, even if the show does adopt that name, but it's interesting to learn what the Duffer Brothers initially envisioned before the project took on a mind of its own.
Now streaming on Netflix, the second season of Stranger Things stars Winona Ryder, David Harbour, Finn Wolfhard, Millie Bobby Brown, Gaten Matarazzo, Caleb McLaughlin, Natalia Dyer, Charlie Heaton, Cara Buono, Matthew Modine, Noah Schnapp, Joe Keery, Sadie Sink and Dacre Montgomery.