Advanced persistent threat [APT] is a military term adapted into the information security context that refers to attacks carried out by nation-states.  APT-related threats are created by a group of developers using in-house tools that are not usually found in the cybercriminal underground.
Difference from targeted attacks
APT is usually mixed-up with the term “targeted attacks” due to the complex nature of both. While targeted attacks also involve complex stages similar to APT, their targets are different; targeted attacks aren't carried out by nation states.
[Related: Targeted Attacks versus APTs: What’s The Difference?]
Meanwhile, APTs are more sophisticated in nature and require deft knowledge and skills to execute. Since APTs are state-sponsored attacks, their operation usually lasts longer. It is also typical of APT attacks to go after a country’s infrastructure, such as its power grids, nuclear reactors, or fuel pipelines.