Originally part of Kevin Hardwick's Narrative Stance Model, later adopted by Ron Edwards as just Stance. The original term meant conscious portrayal of character, as opposed to In-Character Stance which meant acting as character. Ron Edwards' Actor Stance means deciding on the character's decisions and actions using only knowledge and perceptions that the character would have -- but not necessarily speaking in-character or immersing (see Immersion).

References:

1. Kevin Hardwick's "Narrative and Style" (rgfa)
2. Sarah Kahn's "Reiterating the Narrative Stances" (rgfa)
3. John Kim's rgfa FAQ, Part 0
4. Neel Krishnaswami's rgfa Glossary
5. GNS and Other Matters of Role-playing Theory, Chapter 3: Stance
6. Is Director Stance Real? (The Forge, Oct 2001)
7. Further on Stances (The Forge, Oct 2001)
8. Making Stuff Happen non-Stance (The Forge, Nov 2001)
9. Stance Theory: The Hegemony of One Character (The Forge, Oct 2004)