The United States Computer Emergency Readiness Team (CERT) defines a fake anivirus as:
Fake antivirus is malicious software (malware) designed to steal information from unsuspecting users by mimicking legitimate security software. The malware makes numerous system modifications making it extremely difficult to terminate unauthorized activities and remove the program. It also causes realistic, interactive security warnings to be displayed to the computer user.
Read more here: www.us-cert.gov/cas/tips/ST10-001.html