Which terms? What "long standing definitions"? Where are these found? What makes you think you've got better cognitive access to them than I do?
And following on from these questions . . .
According to [MENTION=996]Tony Vargas[/MENTION], the concept of "player agency" was invented at The Forge and means more-or-less what I use it to mean. I don't have my own independent recollection of the use of the term at The Forge - I'm more familiar with their notion of "protagonism", which has a similar (but maybe not identical) meaning.
I've just gone to check
The Forge Provisional Glossary, and found that it generally uses the word "control" rather than "agency" - but it defines
force as
The Technique of control over characters' thematically-significant decisions by anyone who is not the character's player. When Force is applied in a manner which disrupts the Social Contract, the result is Railroading.
No definition is offered of "thematically-significant decision", but "theme" is defined as
The point, message, or key emotional conclusion perceived by an audience member, about a fictional series of events.
Now you insist that
Agency is just the players being able to control the actions of their PCs. I don't disagree with your description
as a description - it entails that when there is
force, players lack agency, and that seems right. (We could quibble over whether "decision" and "action" co-refer, but I'm not going to.)
All the action consists in the following:
what does it mean for a player to control the actions of his/her PC? Or for another participant (such as the GM) to exercise control over those?
My own view - which is not an expression of a semantic opinion, but an expression of a preference for play - is that if a player's declared action cannot succeed, because of an unrevealed decision by the GM about the setting/backstory, then the player
does not have control over his/her PC's actions. The GM has, on that occasion of play, exercised control.
The previous paragraph states a real view - that is, an opinion that I really have. You have a different view, reflecting different RPGing preferences - fine! But that doesn't stop me having, and stating, my view, using English words to express it.
I have some further views, too. If an action declaration doesn't pertain to anything of thematic/dramatic significance, and puts nothing at stake, then sometimes I think it is appropriate for the GM to say "no" and move things on. A paradigm of this, which [MENTION=82106]AbdulAlhazred[/MENTION] mentioned not far upthread and which I think I may have mentioned a long way upthread, is - in my 4e game - searching bodies or rooms for generic loot. That is the sort of no-stake irrelevance that I'm not interested in spending time on at the table, and the alternative to "You find 12 cp" is "No, there's nothing there, now can we get on with it?!"
And here's another one: if the GM is adjudicating action resolutions by reference to a prior conception of the details of the gameworld - whether in the notes, or made up on the spot - then ascertaining those details starts to become a focus of play. Which, per se, means that thematically-significant action declarations becomes less of a focus of play. That makes RPGing less enjoyable for me.
And for fun and completeness, here's one example of how "say 'yes' or roll the dice" can be applied in the context of
thematically significant action declarations in relation to loot: