What does "hack" and "hacker" mean?

Native speaker’s answer
Rebecca
To "[computer] hack" means to use a computer to gain unauthorized access to data in a system. Basically, it's the online version of breaking into a system. A hacker is someone who hacks, and they can be amateurs or professionals. Ex: I hacked into my school system and cancelled all the exams. Ex: Online security is important, as hackers can cause a lot of damage.