This is what Webster's Dictionary has to say:
Definition of GEEK
: a carnival performer often billed as a wild man whose act usually includes biting the head off a live chicken or snake
: a person often of an intellectual bent who is disliked
: an enthusiast or expert especially in a technological field or activity
Examples of GEEK
1. He was a real geek in high school.
Origin of GEEK
probably from English dialect geek, geck fool, from Low German geck, from Middle Low German
First Known Use: 1914
That's all well and good, except maybe that whole biting heads off chickens (EWWWWWWWW!), I know a lot of self proclaimed geeks, and well, none of them are doing that in their spare time, at least not to my knowledge.
The popular definition boils down to (with my interpretation) someone who is enthusiastic and smart about something, and sometimes they annoy people with their passion. That fits my personal definition fairly well, my definition is that a geek is a person who is very passionate about something and with that passion comes the desire to learn about it and celebrate it, and some people may not like that.
I also believe that EVERYONE has a specific brand of nerd, everyone has something that they are passionate about.
For instance My father was REALLY into the History of the Civil War, that was his brand of geek.... He was Passionate, knowledgeable and willing to drag his family to every civil war battle field possible. I remember a trip to Gettysburg PA, when I was about 12 where we got to see a huge reenactment, and he was totally geeking out the entire time we were there.
Another example is my husband, He is more a "stereotypical" Geek, he LOVES computers, he builds them, Plays on them (MMOORPGs)reads about them and attempts (not sucessfully) to teach me how the magical boxes work. (I still think its little fairies and magic smoke lol) He also reads a lot comic books (he's on a first name basis with comic book store owner guy) and has decided Stan Lee is his personal hero.
My Brand of Geek consists of a deep abiding love of Sci-Fi, mostly Star Trek, but also Star Wars, Stargate (mostly just SG1) and Firefly; also the books the Hitchhiker's Guide to the Galaxy. I am also a HUGE fan of the works of J.R.R Tolkien and I have read the Lord of the Rings and the Hobbit more times than I even care to think of. I am also fully capable of a full blown geek out about horses. Thats right, I am a horse geek, I spent the first my first 20 years with them. There are other facets to my geekdom but those are the two major things that get comments from my mother that i should just get NERD tattoo'ed on my forehead.