Here's the definition from www.nolo.com (external - login to view)
The mental component of criminal liability. To be guilty of most crimes (external - login to view), a defendant (external - login to view) must have committed the criminal act (the actus reus (external - login to view)) in a certain mental state (the mens rea). The mens rea of robbery, for example, is the intent to permanently deprive the owner of his property.
By the way, who was it that said it was unethical for an officer of the court (lawyer) to question another officer of the court (police officer)? From reading case law, I can see that police officers get ripped to shreds all of the time in the courts.
There are cases where the police are accused of negligence, assault, maliscious prosecution, use of excessive force, unreasonable search and seizure, you name it. I was just looking at one case where the police were accused of conspiracy to traffic narcotics. Unethical to accuse and question the police in court? My ***!