Inside the community, a hacker (as opposed to "cracker" or "security hacker") is someone who uses technology in a creative way.

My understanding of hacker is specifically someone who exploits vulnerabilities in code. Regular programmers are like building architects, hackers are like people holding up a mask so that the facial recognition powered NEST lock will let them inside the building.

so what do we call undesirables like anonymous and so on?

"crackers" or "security hackers". I'm sure most people on this site would consider themselves to be some form of "hacker", after all, this is "Hacker News".

"cracker" or "security hacker"

Every programmer of course reads some source-code because they must read their own source-code. But such a programmer might use an Open Source library without reading its course. Whereas if you are truly hacker you are interested in how things work and you would more likely be reading such source.

I agree that definition of "hacker" is somewhat vague but mostly people understand it the same way depending on context.

I assume that reason dedicated programmers are called hackers is that earlier the the word "hack" referred to writers.

https://en.wikipedia.org/wiki/Hack_writer

What the hack, there's even a pub called "Old Hack"

https://www.tripadvisor.com/Restaurant_Review-g188644-d10512...

You're actually too far ahead of the times.

http://www.catb.org/jargon/html/