Wikipedia defines it as a person who "mostly plays unusual or eccentric characters". This is why we often hear Johnny Depp, Tilda Swinton, and others called "character actors". These would be thespians who embody a wide variety of characters while performing, their own personalities completely submerged into the fictional creation they are portraying.
However, this is not always the way the term is used. Just as often, you hear "character actor" being used for people like Harry Dean Stanton or Brian Dennehy who are able to essentially slide themselves around any role without losing the qualities that make them so individually unique and without being typecast as one certain kind of persona.
At the same wikipedia page I linked to above, it is interesting to read the differing opinions about this term under the talk tab.
The first entry gives the general definition as I was in the habit of using it:
Character acting occurs when an actor makes a significant physical, vocal, external and/or psychlogical adjustment from the actor's primary persona. This is in contrast to personality acting, where an actor simply uses their habitual persona while they act. Examples of accomplished character actors are Anthony Hopkins, Dustin Hoffman, Lily Tomlin and Denzel Washington. Examples of accomplished personality actors are Harrison Ford, Julia Roberts and Halle Berry.
So what do you think? How should we actually define character actor? And, in doing so, what should we call the other type of actor who can no longer be called a character actor?