Wiki vandalism is generally defined as editing a wiki in a way that is intentionally disruptive or destructive. There are four generally acknowledged types of vandalism: deletion of legitimate information, insertion of noncents or irrelevant content, addition of unwanted commercial links (spam), and policy violations specific to that wiki.
I know it when I see it
Does it really need to be defined?
It's arguable that most (identified) vandalism has consisted of really quite obvious cases. Hence, Wikipedia doesn't need to define an "official" policy on what constitutes vandalism at all. We can use the rule of thumb, "When a reasonable person might be in doubt as to whether something is vandalism, it would be polite not to call it vandalism."
Of course, that depends on the normative definitions of "obvious", "reasonable" and "polite", which are necessarily subjective, and vary from encyclopedia to uncyclopedia.
Patent vandalism is vandalism where both the reader and the contributor agree that it is vandalism. This is sometimes easy to determine (e.g. vandal puts HAHAHAHAH as their edit summary or otherwise states that it is vandalism), and sometimes slightly harder.