Merriam-Webster's definition[1]:

  3: an expert at programming and solving problems with a computer
  4: a person who illegally gains access to and sometimes tampers with information in a computer system

  1. a person who uses computers to gain unauthorized access to data.
  informal
  an enthusiastic and skillful computer programmer or user.

[1] https://www.merriam-webster.com/dictionary/hacker [2] https://www.google.com/search?q=define+hacker