I used to think it was an actor who always played a specific type of character, but then many people called "character actors" have had diverse roles.
I would call a character actor someone who plays secondary, but important roles. Often they're actors who disappear into roles, rather than call attention to themselves as an actor. They're usually not conventionally handsome, but they bring flavour to a role.
I think that wikipedia entry is fucked. It's got a modern idea of character actors confused into the idea of someone who plays a character type all their life, which you'd find more in commedia delle arte, or maybe older studio or repertory systems.
Also, it calls a lot of people character actors who've mostly been leading men: Tom Hanks, Laurence Olivier (?!?!) , Kevin Spacey, Jack Nicholson, Dustin Hoffman. I think Jack Nicholson is the antithesis of a character actor. He's always Jack, doing his thing.