An advanced persistent threat (APT) is a stealthy computer network threat actor, typically a nation state or state-sponsored group, which gains unauthorized access to a computer network and remains undetected for an extended period.

Advanced – Operators behind the threat have a full spectrum of intelligence-gathering techniques at their disposal.

Persistent – Operators give priority to a specific task, rather than opportunistically seeking information for financial or other gain.

Threat – APT’s are a threat because they have both capability and intent.

-Wikipedia