What is it called when one side sends a person to another side to spy on them?

When one side sends a person to another side to spy on them, it is typically referred to as "espionage." The individual sent is known as a "spy" or "intelligence agent," and their mission is to gather confidential information, often related to national security, military operations, or competitive advantages. Espionage can involve various techniques, including infiltration, surveillance, and covert operations.