Vandalism is a problem on all large public wikis. It consists of making a page drastically less useful and reliable for the average end user, typically by inserting profanity or personal comments or provably false statements, "defacing" it, blanking it, reverting it to contain already-corrected errors, or other things that an editor would recognize as "damage". The word should be used only for cases of clear or simple vandalism.
Sometimes removing critical links and references, big blocks of text, or reverts to material which is not false but perhaps controversial is called vandalism, but this is highly political and subject to much systemic bias and other community point of view issues; There are several cases which each need their own terminology:
- Sysop vandalism is systematic bias applied and reinforced by the sysop power structure - it only becomes vandalism when persistent ad hominem deletes or out of process actions occur, e.g. as Wikimedia authorizes
- Ordinary ad hominem revert is sometimes called vandalism, and it is, if and only if errors (not just bias) is restored to user visibility. It only becomes sysop vandalism if IP blocks or protected pages are used to enforce the sysop's view over the non-sysop's view. This is very common in any Wikimedia forum, and sysops are encouraged to express their own biases.
- Sysop vigilantiism and developer vigilantiism are power abuses but they are not vandalism. They are more likely to involve libel against a person than damage to pages themselves, though they prevent some people contributing.
It is a common political tactic to call trolls "vandals", e.g. as Tim Starling and M. R. M. Parrot do, although there is a clear distinction between the two concepts: one tells a vandal based on what ordinary end users would distinguish about the material; a troll on the other hand is a person whose view is merely offensive to someone in a sysop power structure; Both labels are used as simple insults and the use of neither has anything to do with the material's bias or even accuracy necessarily: calling someone "a troll" is a common usurper tactic, e.g. employed heavily by Auntie Angela, Erik Moeller, as a prelude to censorship. Important to realize:
Trolling and vandalism are ontological distinctions, not operational ones - one cannot tell they have occurred based on responses or excuses or vigilantiism that they appear to "cause". It's a social question, and thus must be resolved politically, e.g. by factions.