dragonwriter and others here are both right. In terms of language, the terms are the same, in that you are a criminal if you violate the law. However, in many countries and regions "criminal" can be a way to discern that certain crimes come with specific punishments, like jail time or personal fines.

For example, in the US there's criminal and civil cases. You can't be incarcerated for losing a civil suit, unless you don't pay whatever judgement the court discerns (which is considered criminal contempt of court).

In the US the difference also comes into play in terms of who can bring certain charges against you. If you were to sue someone, you can't sue them for assault and battery or homicide, because these are considered criminal charges and only the District Attorney / the government can bring these charges against someone. In this case, there are typically parallel civil remedies you can sue for, such as "wrongful death" and "emotional distress" which is strictly to recuperate financial loss.

Those remedies are against the parties breaching the contract. It’s a small hair to split, but relevant given the point being made.

contract law is the body of precedent and collective set of laws related to the legality, construction, execution and all around use of contracts, and while I’m sure there are some specific extra laws that make breach of certain types of contracts a misdemeanour or felony. Generally speaking in most jurisdictions around the world in fact, breach of contract is a civil law matter between the parties of the contract, and settlement of the civil law side of the “is this actually a breach of contract” is usually necessary first before any potential criminal liability can be applied.

An example: An contract can be invalidated for lack of sufficient consideration offered (peppercorn isn’t always enough to make it valid)… this makes the contract void, not illegal. This is a simple example but it illustrates the basic point well, and I’m not trying to write a legal theory essay while on the way to work.

"contracts" is civil law. to be illegal it would have to be criminal law. defamation is legal. it's a tort, you can get sued for it and lose. in civil court.

there is contract law, but breaking contract law is not illegal. a tort for example, is a tort, not a crime.

But this isn't a question of logic, it's a question of definition.

And all the discussion I can find online say breaking a contract doesn't fall under the definition of "illegal".

Do you have an authoritative source that defines it otherwise?

Actually no, because contracts are enforceable as long as they don't go against the law, there's a limitation for a contract and contracts cannot be penally prosecuted

Are traffic infractions criminal? Violating some traffic laws can be a misdemeanor or felony while most are infractions. I don’t know if infraction is considered “criminal”.

Almost all states have drinking and driving (DUI/DWI) as misdemeanors and it's a misdemeanor federally.

Name two