It's no secret: espionage is the act of organized spying, usually with the goal of uncovering sensitive military or political information.
If the government has relocated you to a foreign country, made you change your name, given you a fake mustache to wear, and instructed you to hide in a closet at the presidential palace and report back weekly on what you overhear, then you are engaging in espionage. This noun is similar to snooping, but generally applies to much more serious situations, like when countries or businesses spy on each other to try to obtain top secret information.
