Actual cause is the direct catalyst that makes an event take place. Without the actual cause, the event would not happen. For example, say a person smashes a storefront window with a baseball bat. In this circumstance, the person smashing the window with the bat would be the actual cause. The event would be the window getting smashed.
In lawsuits it is necessary to determine what or who caused an event. The reason is because the person who is found responsible can be held legally liable. The "but-for test" is often used to determine actual cause. In a but-for test, an examination is made in which the accused person's actions are analyzed to determine if the event would not have have occurred "but for" the person's actions. The results of the but-for test determine whether or not the accused person can be held responsible for the event