Advanced Persistent Threat (APT)

An Advanced Persistent Threat (APT) is a long-term, sophisticated, multi-vector penetration attempt. APTs are generally directed at high-value military, commercial, or government targets. APTs are generally undertaking as part of state-sponsored or organized criminal espionage.

References:

http://www.schneier.com/blog/archives/2011/11/advanced_persis.html

http://fismapedia.org/index.php?title=NIST_SP_800-39_Appendix_B

http://www.sans.org/reading_room/whitepapers/malicious/detailed-analysis-advanced-persistent-threat-malware_33814