Advanced Persistent Threat - a hacking group that finds an open door (or creates on by phishing), gains access, spreads little backdoors and malicious code into a bunch of machines, any of which, if removed, can be re-created by the others. Not easy to detect, very hard to get rid of once infected. Many groups have differences in the tools they use, which enables those in the know to say "Oh, that's APT X" when they look at an infection.

You may have heard of "Fancy Bear". That's one APT out of Russia.

For more information, see https://en.wikipedia.org/wiki/Advanced_persistent_threat

Advanced Persistent Threat

https://attack.mitre.org/groups/ https://en.wikipedia.org/wiki/Advanced_persistent_threat