There is no real consensus on what a "character actor" is.
A quick scan of definitions on the internet will give you a range of differing opinions on what it means to be a character actor, but there are common elements to them all.
*By standards within the acting community, they are successful in their trade, and respected by their peers.
*True "stardom" has for the most part eluded them, but their talent has gotten them enough work to build a career. They're almost never number 1 on the call sheet, but often land important roles as supporting actors and in the case of television, recurring characters. They sometimes take roles in commercials to supplement their income.
*To me, the most important trait of all, "You'd know them if you saw them." Character actors are seldom known by name, but by their faces and roles.
I'll start this off by giving an example of my own.
He's an Oscar nominee, an Emmy winner and recipient of many film critic awards. His father was a dye maker. His mother was an actor in local community theater and gave him the acting bug. He's been an uneasy ally of Al Swearengen, an ill-fated inmate at a mental institution and the voice of a psychopathic, serial-killing doll.
Submitted for your approval...Brad Dourif.
(Forgive the horrible music in this video.)