Vandalism is any addition, deletion, or change to content made in a deliberate attempt to reduce the quality of The Construction Set Wiki. The most common type of vandalism is the replacement of existing text with obscenities, page blanking, or the insertion of bad jokes or other nonsense. Fortunately, this kind of vandalism is usually easy to spot.
Any good-faith effort to improve The Construction Set Wiki, even if misguided or ill-considered, is not vandalism. Apparent bad-faith edits that do not make their bad-faith nature inarguably explicit are not considered vandalism. For example, adding an opinion once is not vandalism — it's just not helpful, and should be removed or restated.
Committing vandalism is a violation of Standards and Policies; it needs to be spotted, and then dealt with — if you cannot deal with it yourself, you can seek help from others.
Not all vandalism is blatant, nor are all massive or controversial changes vandalism: Careful attention needs to be given to whether the new data or information is right or whether it is vandalism.
(Special thanks to Wikipedia)