Vandalism is the behaviour attributed originally to the Vandals, by the Romans, in respect of culture: ruthless destruction or spoiling of anything beautiful or venerable. The term also includes criminal damage such as graffiti and defacement directed towards any property without permission of the owner.
Vandalism is any addition, removal, or change of content in a deliberate attempt to compromise the integrity of Wikipedia. Examples of typical vandalism are adding irrelevant obscenities and crude humor to a page, illegitimately blanking pages, and inserting patent nonsense into a page. Vandalism is prohibited. While editors are encouraged to warn and educate vandals, warnings are by no means necessary for an administrator to block.
Even if misguided, willfully against consensus, or disruptive, any good-faith effort to improve the encyclopedia is not vandalism. Edit warring over content is not vandalism. Careful consideration may be required to differentiate between edits that are beneficial, detrimental but well-intentioned, and vandalizing. Mislabelling good-faith edits as vandalism can be considered harmful.
Upon their discovery, revert clearly vandalizing edits. Then warn the vandalizing editor. Notify administrators of vandalizing users who persist despite warnings, and administrators should intervene to protect content and prevent further disruption by blocking such users from editing. When warranted, accounts whose main or only use is obvious vandalism or other forbidden activity may be blocked even without warning.
Private citizens commit vandalism when they willfully damage or deface the property of others or the commons. Some vandalism may qualify as culture jamming or sniggling: it is thought by some to be artistic in nature even though carried out illegally or without the property owner's permission. Examples include at least some graffiti art, billboard "liberation" and possibly crop circles. Criminal vandalism takes many forms. Graffiti on public property is common in many inner cities as part of a gang culture; however, other more serious forms of vandalism that may take place during public unrest such as rioting can involve the willful destruction of public and private property. Vandalism per se is sometimes considered one of the less serious common crimes, but it can become quite serious and distressing when committed extensively, violently or as an expression of hatred and intimidation.

en.wikipedia.org/wiki/Wikipedia:Vandalism