What does ISAC stand for?
ISAC stands for "Intelligent System Analytic Computer". It is confirmed at the beginning of the game, as you can see in this playthrough video at 5 minutes and 8 seconds.
It is also confirmed by the tvtropes site :
ISAC, the personal assistant AI used by Agents to give them updates and alerts stands for "Intelligent System Analytic Computer".
The Famitsu's glossary page for The Division also confirm it from what we can read from it :
■ISAC “Intelligent System Analytic Computer”の略称。膨大な情報を一元管理して伝達し、現場のエージェントをつねにサポートしてくれる。