Restricting the distribution of any material on content-based criteria by persons other than sender or intended receiver is censorship.

Whether it is desirable censorship or not is generally a separate issue from whether or not it is censorship, unless, for example, you have previously adopted a rule that the particular actor committing the censorship shall not engage in censorship at all, in which case they are, of course, inherently the same question. (Where this gets hairy is when one likes to pretend that one has such a rule for a particular actor, but actually really would prefer that actor to censor certain things, which sometimes occurs with modern liberal democratic regimes, and especially frequently occurs with a particular North American one which has what superficially looks like a very strong restriction in that area in its Constitution.)