I'd say if you are unfairly depriving someone of an audience that would have wanted to listen to you.

Individually blocking someone you don't personally want to hear from obviously isn't censorship.

But if you have a monopoly on a platform and block everyone who would be interested in listening to someone, just because of your personal preferences, that is censorship.

But if virtually no one wants to listen to something and you block it, I would argue that's not censorship. E.g. no one should has to listen to spam or look at porn.

Of course those lines are blurry, but so is all of moral judgement.

It's more clear cut if you 'unrightfully' ban someone from YouTube, since it's a monopoly. Banning someone from lemmy.world who would have had an audience there is trickier, since ideally this would eventually lead to them and their audience moving to an instance where they are welcome.

That's why you would want your government to protect speech, since it is the biggest and most powerful monopoly. But in my opinion the same should extend to any large institution, like social media.

And I'm talking about censorship as a moral judgement free term, since I would argue there is some good censorship. E.g. banning CSM. I don't think it makes sense to call it anything other than it is.