Meaning
The state of mind of a person accused of crime. This may include the intention / knowledge or forethought of the crime, before the act was committed.
Easy way to remember
Mens = mental condition / of the mind / intention
Rea = reum / the accused / defendent
Explanation
This maxim holds good for criminal law, where not only the act of the accused must be proved, but the intention of the accused to do that specific act must also be proven to show guilt and sentence him for that particular crime.
The act follows the intention.
Only when the act committed and the state of mind of the accused is in concurrence, can he be sentenced for committing a crime.
Common Misconception
This legal term is often misconstrued to mean “guilty mind”. It only means that the mental state of the accused must be in concurrence with his act.
No where is the word guilt mentioned or implied.
Example
An accused who commits a motor vehicle accident which leads to death of the victim, may be charged for murder, if he had the intention of killing the accused i.e. his act and mind worked in unison to execute the crime. Else, it would be considered an accident or negligence attracting a lesser punishment.
For more see: Actus non facit reum nisi mens sit rea