Subversive describes something that tries to attack or damage a system — like a government, a set of rules, or a common belief — often quietly, or from the inside, rather than with open force. A subversive book, joke, or idea makes people question authority, or secretly weakens it. It can be used as an adjective ('a subversive film') or, less often, as a noun for a person who does this ('the police arrested several subversives'). It's a fairly formal word, and it can carry disapproval (from the point of view of the authority) or admiration (for someone bravely challenging power).