An advanced persistent threat (APT) is a prolonged, aimed attack on a specific target with the intention to compromise their system and gain information from or about that target. The word threat doesnt mean to imply that there is only one kind of malware involved, because an APT usually consists of several different attacks. It is not uncommon to see the use of unpatched vulnerabilities (zero-days) in this kind of operations. The tasks in the list can be very different in time and the amount of effort, spent on them.”]
Source: https://blog.malwarebytes.com/101/2016/07/explained-advanced-persistent-threat-apt/