It is one of those buzzy political terms beloved of those who have watched too many reruns of The West Wing. One need not be Jed Bartlet or CJ Cregg to know what it means: an event that tips the balance heavily in favour of one side or another. In reality, such is the multifaceted nature of modern campaigning, game changers are about as rare as vegan lions.
But in JK Rowling's "no" declaration this week, Scotland's independence debate may just have found one. If this is the moment that brings clarity to the campaign it has been a long time coming. The longest political exercise of our lifetimes has involved trekking through blizzards of statistics and hiking our way up mountains of pronouncements. And that is before the campaign proper even starts.
There have been many false calls on the game changer front. The Scottish Government's publication of its White Paper was meant to be a game changer, but it turned out to merely reflect the state of play as everyone knew it. The tripartite declaration from Messrs Osborne, Balls and Alexander against a currency union with an independent Scotland was supposed to be another. Alas, the balloon rather burst on that one when an unnamed minister reckoned this was just another negotiable.
Then there was the intervention of President Obama this time last week. Having studiously kept out of the debate in public, No Drama Obama was initially thought to have caused a right stushie by declaring that the Union was working perfectly well and there was no reason he could see why anyone would want to change that.
It ain't broke, so why fix it? While it should have been a game changing moment, and it certainly broke new ground in the public conduct of diplomacy, any surprise there might have been was soon followed by a shrug. Was there ever any doubt as to which side the US would back?
Put it down to our historic ties, our trade alliances, our pretty blue eyes, or those weapons of mass destruction parked at Faslane, but there was no way the US was going to stay out of this one.
The Yes camp should, and certainly claimed it did, see the Obama declaration coming. It might say the same about JK Rowling's intervention and donation of £1 million to Better Together. Though she had been careful not to make any obvious pronouncement until this week, the fact that the Harry Potter author is friends with Gordon and Sarah Brown, and has donated to the Labour Party, made it highly likely that it would be a matter of when, not if, she would make her choice known.
Even so, the manner and timing of her intervention are crucial. Rowling declared her hand at a time when there is genuine concern over how high the political temperature is rising, particularly on the internet. Far from being cowed by critics, she got her retaliation in first, calling any attempt to question her Scottishness "Death Eaterish". Lord Voldemort himself would have smiled at that pre-emptive strike.
Then there was the message itself, which managed to be partisan while at the same time appearing even-handed. Never mind The West Wing. If any political anorak wants a masterclass in effective communication, they should read Rowling's mini-essay on why she is supporting Better Together. For a start, there is not one statistic in it, no "they said this, but the other side said that". It is all about conveying a general sense of scepticism, of taking what the Yes camp says in good faith, then quietly demolishing it.
While her words will not convert a Yes voter to a No or vice versa, her conclusion will resonate with the don't knows. When she writes, "While independence might give us opportunities -any change brings opportunities - it also carries serious risks", she is crystallising the concerns of doubters more effectively than any politician so far. Important as her message is, it is not half as significant as the messenger. Doubtless there will be some who see Rowling's intervention as no more significant than that of any other celebrity, and that it matters not a jot what she, or Sir Sean, or Alan Cumming, or David Bowie, think about independence.
Might as well convene a seance and ask Elvis what he reckons, or flip a coin, than place much store by the opinions of those in the entertainment business. And Rowling is, after all, in the business of show.
She is not a politician. She has never, as far as is known, stood for election to political office in her life. She is where she is due to the success of her books for children. She is Harry Potter's mum, for gawd's sake, not The Oracle. One would not ask a teacher to run a supermarket, or a heart surgeon to fix a car, so why take what Rowling says on the issues of the day as gospel?
All true, but with Rowling one cannot help but think that there is a difference. Given her standing in Scotland, and what she represents, the Yes camp would be foolish to think this is a mere bump in the road, something easily navigated around. If it reckons that her missive is just another celebrity intervention, something to be forgotten as quickly as the name of the latest Britain's Got Talent winner, it should think again.
At this point in the campaign there is probably no such thing as a game changer when it comes to confirmed Yes and No votes. Those who have made their choice did so long ago and are not for turning.
But when it comes to the don't knows, different rules apply. For those still swithering, those who continue to look for signs and wonders, the position JK Rowling takes does have some significance.
This referendum was always going to be decided on two questions: who we think we are, and who we think we can be. On both those counts, Rowling's experiences will hold a lot of sway. Here was someone who became a success through her own talent and hard work.
A person who faced difficult times as a single mother but made her way through them. Someone who has had global success, who has improved lives (reluctant-to-read boys in particular), who made a vast fortune, and who gives sizeable amounts of it away to help others.
Smart, talented, world-beating, caring, resourceful; everything we want modern Scotland to be.
When it comes to presenting an attractive face of Scotland there is one more thing to note about Rowling. It is a fact she touched on herself. She was not born here.
She is Scottish by choice. With her wealth she could live anywhere she wants, but she plans to remain here for the rest of her life. That is a pretty solid vote of confidence in the status quo.
Perhaps you remain unconvinced. Maybe the game did not change on Wednesday. But ask yourself where the polls would stand next week if Ms Rowling had come out in favour of independence. Harry Potter and the Referendum Touchstone. That is one bestseller the First Minister could do without.