Dictionary Definition

Slang.
–noun
1. a peculiar or otherwise dislikable person, esp. one who is perceived to be overly intellectual.
2. a computer expert or enthusiast (a term of pride as self-reference, but often considered offensive when used by outsiders.)
3. a carnival performer who performs sensationally morbid or disgusting acts, as biting off the head of a live chicken.
- Dictionary.com

What Wikipedia Says

The definition of geek has changed considerably over time, and there is no longer a definitive meaning. The terms nerd, dweeb and dork have similar meanings as geek, but many choose to identify different connotations amongst these terms, although the differences are disputed. In a 2007 interview on The Colbert Report, Richard Clarke said the difference between nerds and geeks is "geeks get it done".[3] Julie Smith defined a geek as "a bright young man turned inward, poorly socialized, who felt so little kinship with his own planet that he routinely traveled to the ones invented by his favorite authors, who thought of that secret, dreamy place his computer took him to as cyberspace—somewhere exciting, a place more real than his own life, a land he could conquer, not a drab teenager's room in his parents' house".[4]

Other definitions include: