- Geek: Understands, creates, and fixes Really Cool Stuff.
- Nerd: Understands and collects Really Cool Stuff.
- Dork: Confused by Really Cool Stuff.
- [?]: creates Really Cool Stuff by cleverly combining things nobody else would have thought to put together.
That word is "hacker."
To find out what "hacker" originally meant, you should spend a while reading the original Hacker's Dictionary, also known as the Jargon File. The terminology and the culture that used it grew up among universities like Stanford, MIT, Carnegie-Mellon (where I went to grad school) and others connected via the ancient ARPAnet. The primary meaning of "hacker" was "a person who enjoys learning the details of programming systems and how to stretch their capabilities, as opposed to most users who prefer to learn only the minimum necessary" -- #1 in the Jargon file definition list. A hacker could figure out how to combine features most people wouldn't have thought of putting together to get something much smaller and often more elegant than most people would do, often using the features of their programming language more effectively than people who program the same way in every language.
The modern meaning arose because some hackers used their skills to break security on their own or other people's computers. This was rarely malicious initially; doing something clever was the main point. Hackers that did damage were the only ones the public ever heard about.
"Hack" in an older real-world meaning was about doing something quickly rather than well -- a "hack writer" being the archetypical example. You can see the same sort of things in There I Fixed It, about hack repairs. There was some of that in the hacker culture, but people who did so weren't regarded as real hackers. The essence was cleverness and elegance.
The relevance to geekdom in general is that you can be a hacker in any subject. I once worked for a startup company. Since ordinary real-world programmer salaries were more than most of us had ever earned, management arranged for a couple of tax accountants give a seminar on money management and minimizing taxes (legally). At one point they talked about some moderately complex scheme involving putting together a few different sections of the tax code which, on the surface, had little to do with each other, in a way that would save a lot of money. One of my colleagues leaned over and whispered to me, "These guys are tax hackers."
So, if you're an expert in any subject, and love to put together clever artifacts of that subject, you too, could be a hacker. As it is, given the current negative connotations, you probably have to make do with calling yourself a <subject name>-geek.