A lot of confusion arises from the way “Brexit” has come to describe two very different things. It can be heroic liberation from foreign control – a common British definition. Or it can be the task of dismantling the UK’s membership of the European Union, in which heroism plays no part. That is how it is generally understood abroad.

They sound the same and are spelt the same way, but the meanings have diverged so far that really they should be separate words. One is an event, the other is a process. One is booked for 29 March 2019; the other will drag on for a decade or more. One is a fantasy, the other is a negotiation. Fantasies tend to be non-negotiable.

On Tuesday, the Commons starts voting on amendments to the EU withdrawal bill and the debate will be passionate, invoking democracy and patriotism. “Brexit” will mostly be used in its fantastical sense. But some rebellious Tories and many Labour MPs will deploy the B-word according to international usage, speaking the language of negotiable agreement. (That is also what Theresa May will have to do at a Brussels summit at the end of June.)

You hear talk of 'no deal' – but only from those who see setting fire to their head as an alternative to the hairdresser

To understand the difference, it helps to identify the moment when the two meanings parted company. It was 8 December 2017. May presented the “joint report” that closed the first phase of separation talks. To seal the deal, the prime minister promised the EU an invisible border between the UK and the Republic of Ireland. She also promised Arlene Foster, leader of the Democratic Unionist party, that there would be no new differences in the arrangements covering Northern Ireland and mainland Britain. The only way to satisfy both pledges at once is for the UK to retain near-total regulatory alignment with the EU for the foreseeable future. There may have been some ambiguity lurking in the tortured official syntax but, in essence, May signed off on a soft Brexit. That was six months ago. The ink is dry.

If Jacob Rees-Mogg or Boris Johnson believed half of what they have said about ditching the talks, threatening to go it alone on “WTO rules”, they would have rejected the joint report and demanded May’s head. They didn’t, and so their bluff was called. You do sometimes still hear chat about “no deal”, but only from people who would see setting fire to their own heads as a viable alternative to the hairdresser.

Yet the Tories did not accept the consequences of their actions in December. They still think it should be possible to ditch EU regulations without disrupting trade. They want UK enterprises to be part of a single European commercial space, competing with EU businesses, but with a right to selectively apply EU law. No jurisdiction in the world would offer that privilege. Brussels said as much. But off went the cabinet, hunting for magical ways to restore a border without border controls (“customs partnership”), or to reintroduce border controls without a border (“maximum facilitation”).

None of this made sense outside the UK. In domestic debate, the word “Brexit” was still being used nebulously and dishonestly, as it had been by the leave campaign. In Brussels its definition was refined by the joint report: it meant sticking to the deal. The Tories have wasted 2018 so far arguing about deals with a Europe of their imaginations, as opposed to dealing with the one that exists in law. The parameters of available agreement are known. May could withdraw mainland Britain from the customs union and single market – economically potty, but technically possible – while submitting Northern Ireland to joint jurisdiction with the EU. She would challenge the DUP to suck it up because if they brought the government down they might wake up to find Jeremy Corbyn (whom they dread as an agent of Irish reunification) in Downing Street.

Or, she could sign the whole of the UK up to a customs union and single market membership. That would entail submission to European court rulings, payment into EU budgets and free movement of people. It would be just like EU membership but worse, because there would be no top-table seat to defend national interests. If she plays her diplomatic cards very well, the prime minister might upgrade that package to include a consultative role when big decisions are being made, plus a modest dispensation on free movement. Skirting the boundary between realism and fantasy is a proposal to stay in the single market for goods only – allowing a more independent trade policy in services. Ambitious; maybe not impossible.

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Those are the runways on which the Brexit plane can land. It is circling in the air because May doesn’t want to tell the passengers about the destination. Wisely, she rejected advice from her backbenchers to ditch the plane in the sea or slam it into a cliff. The point of departure now feels very remote. Even if she wanted to go back, it isn’t clear whether there is enough goodwill in the EU tank to get us there. Time is the fuel and she is running low.

And May is alone in the cockpit. No one else wants to make the announcement: fasten your safety belts, we are starting our descent. The days when Brexit meant gazing out of the window at wispy cloud castles are over. Corbyn doesn’t want to say it. Johnson can’t say it. They don’t want to tell the public that the options are either: a deal worse than membership or no deal, which is worse than the worst deal.

There is orderly arrival in a second-rate location, or there is a fiery crash-landing. That is what it means to honour the referendum. That is what it meant all along. There is no sunnier destination just over the horizon. But there is also no political leader with the courage to admit it. Instead we have our prime minister, afraid of the runway, without a clue how to turn the plane around, flying on empty.

• Rafael Behr is a Guardian staff columnist