An Advanced Persistent Threat (APT) is a threat involving a cyber attack over an extended period of time using a variety of methods.
These kinds of cyber attacks often go undetected as the attackers are very strategic, basing their decisions on gathered intelligence and leaving little-to-no trace. APT groups are often open about their successes online, and groups are often named in news articles.
APTs are sometimes nation state or state-sponsored attacks, and often target large enterprises including operators of critical national infrastructure (CNI), and advanced manufacturers such as pharmaceutical or automotive manufacturers.