In moral theology, an act is the knowing choice of a human person. Each knowing choice is an act, and each act is subject to the eternal moral law. Some acts are moral, and other acts are immoral. An immoral act is a sinful act. Sin is a knowingly chosen immoral act.
The morality of any act is based on three fonts (or sources):
(1) the intention or purpose for which the act is done,
(2) the inherent moral meaning of the act as determined by its moral object,
(3) the circumstances of the act, especially the consequences.
To be moral, each and every act must have three good fonts of morality. The intention must be good, the moral object must be good, and the good consequences must outweigh any bad consequences. If any one font is bad, the act is immoral. If an act is immoral due to a bad intention, the same type of act may be moral with a good intention. If an act is immoral due to the circumstances, the same type of act may be moral in different circumstances.
But when an act has an evil moral object, the act is inherently immoral, in other words, the act is evil, in and of itself, apart from intention and circumstances. Every intrinsically evil act has an inherent moral meaning (the moral species) which is contrary to the moral law of God. Intrinsically evil acts are never justified by intention or circumstances because the moral species (the type of act in terms of morality) is inherently unjust.
Pope John Paul II: "But the negative moral precepts, those prohibiting certain concrete actions or kinds of behavior as intrinsically evil, do not allow for any legitimate exception. They do not leave room, in any morally acceptable way, for the "creativity" of any contrary determination whatsoever. Once the moral species of an action prohibited by a universal rule is concretely recognized, the only morally good act is that of obeying the moral law and of refraining from the action which it forbids." (Veritatis Splendor, n. 67).
Intrinsically evil acts are always immoral, and are never justified by intention, or by circumstances, or by other knowingly chosen acts.