1. Class. Myth.a gigantic hollow wooden horse, left by the Greeks upon their pretended abandonment of the siege of Troy. The Trojans took it into Troy and Greek soldiers concealed in the horse opened the gates to the Greek army at night and conquered the city.
2. a person or thing intended to undermine or destroy from within.
3. a nonreplicating computer program planted illegally in another program to do damage locally when the software is activated.