A "criminal" is a person who has committed a crime, or who has been convicted of a crime by a court. It can also refer to something related to crime, or involving or being a crime.
Here's a more detailed breakdown:
Criminal as a Noun:
Person who commits a crime:
This is the most common usage. It refers to an individual who has engaged in an act prohibited by law and punishable by the state.
Person convicted of a crime:
This refers to someone who has been formally found guilty of a crime by a court of law.
Criminal as an Adjective:
Relating to crime: It can describe something that is connected to or involves crime.
Of or relating to the prosecution of crime: This can refer to criminal law, criminal procedure, or criminal courts.
Involving or being a crime: This describes an action or conduct that is considered a crime.