Wikipedia:Vandalism

Vandalism is any addition, removal, or change of content made in a deliberate attempt to compromise the integrity of Wikipedia. Vandalism cannot and will not be tolerated. Common types of vandalism are the addition of obscenities or crude humor, page blanking, and the insertion of nonsense into articles.

Even if misguided or ill-considered, any good-faith effort to improve the encyclopedia is not vandalism. Even harmful edits that are not explicitly made in bad faith are not vandalism. For example, although reinserting it despite multiple warnings can be disruptive, adding a controversial personal opinion to an article is not vandalism. However, edits/reverts over a content dispute are never vandalism, but edit warring. Not all vandalism is obvious, nor are all massive or controversial changes vandalism. Careful thought may be needed to decide whether changes made are beneficial, detrimental but well-intended, or outright vandalism.

Committing vandalism violates The Wikipedia policy. If you find that another user has vandalized Wikipedia you should revert these changes; you may also warn the user (see below for specific instructions). Users who vandalize Wikipedia repeatedly, despite warnings to stop, should be reported to Wikipedia:Administrator intervention against vandalism, and administrators may block them. Note that warnings are not always required: accounts whose main or only use is obvious vandalism or other forbidden activity may be blocked without warning.