|The Big Model|
Easy enough: when you're playing a character, you talk in his or her voice and otherwise communicate as if you were them.
Except, not that easy after all, is it? For instance, acting only upon knowledge the character would have in the fiction is sometimes called "in-character" too. Or making decisions in ways someone else at the table doesn't like can get marginalized because you're not being "in-character" enough.
Sneaky, sneaky gamer talk ... it's probably more honest to say that people use the term "in-character" in lots of different ways, toward their own ends of looking like good-and-true role-players.
Not that any of the listed meanings are bad things. They aren't. But they do not, themselves, define or constitute role-playing as an activity.