What do you call it when someone steals a company secret to sell to competitors?

industrial espionage

You might call it "theft of trade secrets for unfair competition". It may be a criminal offense, but is more often a civil violation of a contract with an employee or former employee.

Corporate malfeasance.

Intellectual copyright theft

Corporate Espionage. It actually happens; the, "Capacitor Plague," is an example of it.

Industrial espionage.

theft of intellectual property, corporate espionage, industrial espionage

Industrial Espionage.

Competitive intelligence.

Industrial espionage.

Industrial espionage. You can get in big trouble for that, especially if you signed a Non Disclosure Agreement.