Is hacking illegal?

Yes, hacking without a permission is illegal. Hacking is a term to define breaking into others people computer network without the knowledge of the owner with or without bad intention.

In the other hand, hacking legally means breaking into computer systems with the permission of the owner. This will be done when an organisation wanted to test their network for vulnerabilities and fix it before a hacker will do it in the same way.In the IT industry this is call the penetration testing.