“Cult” is a word that doesn’t have a real precise meaning, and isn’t used by people who study religions and the psychology of religion for a living. They use the term New Religious Movement to describe, well, new(ish) religious movements.
That being said, laypeople use the word cult all the time, but they mostly use it to mean, “this group that believes weird shit that’s different than the weird shit I believe.” So, sure, it’s a cult if you’re not a Scientologist.
If you’re asking if they’re secretive (“occult” in religious studies terms), then yes, sort of. There are lots of ex-Scientologists who are perfectly happy to share their “secrets,” but in order to get much official information out of the actual group, you’ve got to become a member.
If you’re asking if they’re coercive, then yes, again, sort of. If you have money, or have means of making money, and they think they can get you to give it to them, then they do make it difficult to leave, with a combination of blaming the apostate (“If you worked through your concerns using our methods, you’d reach a new level of understanding. If you leave, you’re a filthy stinking quitter!”) and, supposedly, working to destroy their careers by badmouthing them to other Scientologists in their field.
If you’re asking if they force members to cut off all ties with non-Scientologists, then no. They do encourage members to recruit their friends and family as new members, but you’re still allowed to go to Mom’s house for Christmas dinner, talk to reporters, have friends who aren’t Scientologists… There are some New Religious Movements which control their members’ lives so much that you just don’t do that. Scientology is not one of them.
If you’re asking if they brainwash people, then no. But since “brainwashing” as we were scared of it during the Satanic Panic of the '80s doesn’t actually exist, then it’s arguable that no New Religious Movement/Cult brainwashes.