Sailor Moon's group of transforming fighters go by many names. Fans of the 90's English dub call them Sailor Scouts. If you watched the original dub, you probably call them Sailor Senshi. In the series, they're called guardians of their respective planets. But what's the right title?
Let's start with the original Japanese title of the manga: 美少女戦士セーラームーン (Bishoujo Senshi Se-Ra-Mu-N), which calls the team senshi. In English, 戦士 (senshi) translates to warrior, combatant or soldier, according to Jisho.org. Literally translated into English, the title is "Beautiful Girl Soldier Sailor Moon." So that's where "senshi" and "soldier" come from. But what about scout?
Scout was first used in the '90's English dub of the anime. This was most likely in reference to their sailor uniforms, which were modeled off real-life middle school uniforms. The uniforms were first introduced in 1920 in Fukuoka Prefecture by Principal Elizabeth Lee, after she was inspired by the British Royal Navy uniforms. Lee had been to Britain as part of an exchange program when she was a student. The term sailor "fuku" stems from this. There is a branch of the British Army called the Lovat Scouts, but it's unclear if this was the origin of the word in place of senshi. It could be they got it from "Girl Scouts," a group of girls wearing cute uniforms and trying to help others, so the connection could be made. The answer isn't 100% clear.
Guardian is a bit easier, though. The Senshi watch over the galaxy, making them guardians. They protect the galaxy with their powers, and are also guardians of certain ideals. Mars is Guardian of Passion, Venus is Guardian of Love, Mercury is Guardian of Knowledge and Jupiter is the Guardian of Courage. These ideals are most likely in reference to the Roman Gods that the planets are named after. Mercury, for example, is also known as Hermes, God of Communication and Interpreters and the most clever of the gods. Pluto was also a Guardian of the Space-Time Door, and we see her at her post during the Black Moon Arc.
Not only this, but their original incarnations watched over the princess of the Moon Kingdom, so they were guardians over her as well. Sailor Moon, as that princess, protects the Silver Crystal, making her its guardian. Then she goes on to become Neo-Queen Serenity of Crystal Tokyo, and a queen's job is to protect her kingdom. Point being, guardian is an accurate term to use.
So technically, all of these terms are correct. It just comes down to preference, and they are interchangeable. They all mean the same thing, with the exception of scout. Scout is more of a '90's fan term, but that doesn't make it any less viable. There's really no truly correct term because they're all correct. The girls fight, making them soldiers. They watch over the galaxy, making them guardians. No one term is better than the rest. So, if you prefer calling them Sailor Senshi, Sailor Scouts, Sailor Guardians, even Sailor Soldiers, go ahead. You're not wrong.