arhh I define "geek" by passion, not expertise. You're an geek if you do a lot of spare-time reading about the subject, if you'll neep about it in person or online, if you'll listen happily to people who know more than you do, if you have Strong Opinions.
I am a geek about:
House MD
Hugh Laurie
John Barrowman
New Who-verse (Who, Torchwood, Sarah Jane Adventures)
Life On Mars
Old movies.
Musical Theater
Pop music, both British and American, but only through the 80's.
60's/70's/80's Television.
Writing
Fanfic
Fandom
Pop culture in general, both British and American.
American politics
American History
Karaoke
In the past I have geeked about:
Star Trek-Original series
Science fiction
Literature
British politics.
Twin Peaks
Miami Vice
Football (American)
Professional cycling
Acting
Things I might geek about if I had more time/energy, that is, stuff I'm enthusiastic about but not sufficiently well read or practiced in:
Classical music
Cooking
Art
What are you a geek about?