A more thorough explanation:

Definition: A mercenary is a professional soldier who is hired by someone other than their own government to fight in a foreign country. This is according to international law.

Example: A private military company hires soldiers to protect oil fields in a foreign country. These soldiers are not part of the country's military, but are instead hired guns. They are considered mercenaries.

Explanation: The example illustrates the definition of a mercenary because the soldiers are hired by a private company, not their own government, to fight in a foreign country. They are not fighting for a cause or ideology, but for money. This is what makes them mercenaries.