Antonyms for the term "unlawful act" would include words like "legal," "authorized," "permissible," "legitimate," and "lawful." These terms imply that an action has been sanctioned or accepted by the law, and is therefore not subject to punishment or censure. Other related antonyms might include "valid," "proper," "acceptable," and "law-abiding," which convey the idea that an action is within the bounds of what is considered proper or acceptable behavior in a given context. Ultimately, the use of an antonym for "unlawful act" will depend on the specific context of the action in question, as well as the legal and social norms that govern it.