A spy is a person whose job is to obtain secret or confidential information (intelligence) about the activities or intentions of a rival government or group in support of their own government or organization. They are not professional intelligence officers and do not usually receive formal training, but may be taught basic tradecraft. Instead, a spy either volunteers or is recruited to help steal information, motivated by ideology, patriotism, money, or by a host of other reasons, from blackmail to love. From an intelligence perspective, their most important quality is having access to valuable information.

The term "spy" can also be used more broadly to refer to anyone or anything connected to "spy agencies" or any activity done secretly. A person who commits espionage is called an espionage agent or spy. The act of espionage is clandestine, as it is by definition unwelcome.

In summary, a spy is a person who secretly gathers information about a rival government or group in support of their own government or organization.