To be guilty of a crime, a defendant must have acted voluntarily. Voluntary conduct refers to consciously chosen movements and actions. Involuntary behavior, on the other hand, refers to acts over which the actor has no control, such as those that result from reflex, convulsion, or unconsciousness.
For example, suppose Frank is on trial for pushing Jolene to her death from a cliff. If he meant to push her off the cliff, then he acted voluntarily. But if, at the moment he pushed her, he was suffering from a seizure that deprived him of control over his body, then his conduct wasn’t voluntary; unless there’s an indication that he disregarded a substantial risk that he would involuntarily knock Jolene off the cliff (perhaps by ignoring his history of similar seizures), then he’s probably guilty of no crime. (See What amounts to recklessness? and What is criminal negligence?)
There’s an important distinction between voluntariness and intent. A defendant’s behavior can be voluntary even if he or she doesn’t intend the result that occurs. For example, say Frank pushed Jolene in jest, not realizing how close the edge of the cliff was. His argument would be that he didn’t intend to kill or harm her, not that he didn’t choose to push her.
In most cases, voluntariness isn’t at issue because there’s no doubt that the defendant had control over his or her actions. But when the circumstances of the alleged crime suggest that those actions could have being involuntary, then the prosecution must prove beyond a reasonable doubt that they weren't.