The unlawfulness of an act will be the conduct of the perpetrator. It will either be the positive conduct “commission” or negative conduct “omission” that would be a transgression of the law. Commission is when you act willfully in a positive manner against a certain law that prohibits your action. You will be guilty of contravening the law when your commission is unlawful. We can use assault as a good example, or theft of cash. Omission is your conduct where you fail to act when you were supposed to act as set out in a specific law. This means it is to neglect a prescribed act of the law. Here we can think of a mother neglecting to care for her sick child, or a person failing to submit his revenue to SARS when it is obligated to do so.
The act of your conduct means the type of act described in the definitional elements.
Criminal law does not prohibit a mere act in /abstract to /. Put differently, there is no rule of law declaring “you may not act”. At every conscious moment of a person’s existence he/she performs some act or another, such as walking, opening a door, or simply sitting and staring etc.
It stands to reason that “act” as the word is use in criminal law does not refer to the “events” just mentioned, it refers only to the type of act mentioned in definition of the crime with which X is charged, an more specifically the type of act set out in the definitional elements of the relevant crime. The law does not concern itself with any other possible “act” committed by X (i.e. an act other than the one mentioned in the definitional elements). Thus if X is charged with arson, the act required- is setting a fire to a certain type of structure.