In order to define what a criminal is, we first need to define what a crime is.
I'm sure most of us would consider crime something that other people do. But let me put forward a couple of questions to you; have you ever used the work telephone to make personal calls? Have you ever kept money if you received too much change? Have you taken "souvenirs" from a hotel? Have you ever taken stationary or anything else from the workplace? Have you ever found something, money or an object and kept it? Each and every one of these carries a fine and prison sentence that can be imposed if you were caught! Do you now consider yourself a criminal having committed a crime? Based on these examples, things that people often do without thinking and consider "perks of the job", crime rather than an abnormal minority is actually more widespread than we would probably imagine.
The legal definition of a crime is "acts that break the laws of the land" i.e. burglary or murder. These are the sorts of crimes that most readily come to mind. But does this cover all acts that cause injury to the public such as disastrous economic decisions made by the government? Does the law cover all the sins of the world? Is it always a crime to take another person's life? Does this law cover war? Crimes that break a set of norms, moral codes and ethics are known as normative crimes. For example an orthodox Muslim drinking alcohol is considered criminal by his social group but it does not break any legal laws. Legal and normative crimes generally do not agree.
The meaning of crime is a social construction. What this means in simple terms is that the meaning is not determinable by the general use of the word in any given society. The time in history and the social surroundings need to be taken in to account. Very often one person's moral codes conflict with another's. For example a person with an illness and in constant pain, obtains cannabis and smokes it for pain relief. This is against the law but would you consider it a crime?
So basically a criminal is someone who commits a crime. A crime in general everyday use of the word is an act that breaks the written laws of the land and is punishable accordingly. This is the definition that will be used by most people.
To be completely accurate we should say that a criminal is someone who breaks the written laws of the land or the moral and ethical codes of the society he has chosen to live in.