Definition of “hacker” - Learner’s Dictionary

hacker

noun [ C ] uk us /ˈhækər/
COMPUTER

someone who illegally gets into someone else's computer system:

The hacker obtained names and e-mail accounts of the company's one million customers.
TELEPHONE

someone who illegally and secretly listens to someone else's telephone conversations and telephone messages:

The mobile phone hackers face five years in jail.

(Definition of “hacker” from the Cambridge Learner's Dictionary © Cambridge University Press)