A criminal, according to Black's Law Dictionary, is anyone one "who has committed a criminal offense (a crime). A crime (again according to Black's) is "a positive or negative act in violation of (the) law." A "law" in the general sense is "a body of rules of action or conduct prescribed by controlling authority" So whoever is the "controlling authority," to a large extent gets to define who is a criminal.
From the time we learn the word "no!" from our parents, each of us learns what we must do (or not do) to stay in the good graces of our community. It is called "socialization." We learn through various agencies (home, school, church, etc.) to respect the personal and property rights of others and, among other things, to behave in a way that does not endanger the health, welfare, or safety of those around us.
So the person who commits a crime is the person who behaves in an essentially antisocial way, and the penalty is usually prescribed in our code of criminal law. A person who is convicted of a crime, then, becomes a "criminal" in the legal sense of the word.
There is another more subjective category of "criminal." That would be a person who breaks a law that might be deemed unjust of repressive. Examples of such laws would be the racial segregation laws of the old South. Examples of criminals under those statutes would be Martin Luther King, Jr., whose most famous treatise of civil disobedience was written from inside the Birmingham, Alabama, jail.
Forming a bridge between the criminal and the dissenter would be those who choose to commit violent crimes in furtherance of a political cause. The Minutemen of Lexington and Concord and the street fighters of Palestine could be branded either as lawbreakers or heroes, depending on one's point of view. Likewise, had the Revolutionary War or World War II turned out differently, George Washington and Winston Churchill could have turned out to be famous war criminals.
The notion of who is a criminal also evolves as the values and attitudes of a society change. Before the Civil War, in some states it was against the law to teach a slave to read. In Tennessee in 1926 it was illegal to teach Darwin's Theory of Evolution. Until the Supreme Court ruled otherwise, as recently as 1973 in many states a woman could not get a legal abortion. So, whoever broke those laws at that point in time met the definition of "criminal."
So the definition of who is a criminal falls into the generally accepted notion that criminals are lawbreakers who violate the "social contract" requiring good behavior and consideration of the rights of others. But history teaches us that one person's criminal might be regarded as another's hero. Yesterday's "criminal" could be today's law-abiding citizen.
Just as winners of wars tend to write the history we learn in school, prominent members of our society - judges, police, the clergy - (the "controlling authorities" alluded to above) also get to write our laws. Those who write the laws also get to define who is and who is not a criminal.