间谍 (jiàn dié): สายลับ

During the war, he was sent to an enemy country on a secret mission; in fact, he was a well-trained spy.

After a long investigation, the police finally captured a spy who had been hiding inside the government.

This company is suspected of stealing trade secrets; the boss is suspected of being a spy sent by a competitor.

In movies, spies can always cleverly shake off surveillance, but in reality it's not that easy.

She discovered that her husband secretly sent encrypted emails every night and began to suspect he might be a foreign spy.