A computer that is infected with a virus or Trojan horse that puts it under the remote control of an online hijacker. The hijacker uses a zombie to generate spam or launch denial of service attacks. (McAfee Labs – Threat Glossary).

Source: Terms & Definitions of Interest for DoD Counterintelligence Professionals, Office of the National Counterintelligence, https://www.dni.gov/files/NCSC/documents/ci/CI_Glossary.pdf