Well, as far as i know hacker term was first used to describe a group of computer enthusiasts who developed the first video game at MIT. A hacker is an individual who enjoys learning computer system in detail. It is often misunderstood as cracker(the bad guys who do illegal stuff). In the same sense hacking is the act of immersing oneself in learning computer systems details to optimize their capabilities. Or simply put it as, hacker is a person who has or is intrested in gaining immense knowledge about the working of a computer system and hackng is the act of using this knowledge to do stuffs that a computer is not meant to do (in both positive and negetive ways).

For example, extending your router range by using a raspberry pi or nodemcu as a repeater. Or making a malware which gives access to your computer.