At the moment, no-one knows. And what little we do know is in a state of flux due to the UK Parliamentary elections. As manifestos are published, the definition of a "bad deal" may change.
The only clear indicators that we have are from a speech given by Prime Minister Theresa May in January. In this speech, she set out 12 objectives for the UK in the upcoming negotiations. They were:
- Control of our own laws
- Strengthen the Union
- Maintain the Common Travel Area with Ireland
- Control of immigration
- Rights for EU nationals in Britain, and British nationals in the EU
- Protect workers' rights
- Free trade with European markets
- New trade agreements with other countries
- The best place for science and innovation
- Cooperation in the fight against crime and terrorism
- A smooth, orderly Brexit
However, there are several problems in defining a "bad deal" against these terms.
Firstly, some of them (see points 7, 11 and 12 for example) are either wooly or double-edged. We cannot make a judgement as to what settlement in these areas would constitue "bad".
Second, other statements from the government suggest that some of these goals are more important than others. Control of immigration, for instance is seen as a key driver behind the vote to leave and thus essential for getting a "good" Brexit. New trade deals with other countries is also seen as a critical path to avoiding economic damage from the divorce.
Third, many of these goals appear to be incompatible with each other, if statements from the EU are to be taken at face value. The commission has said that the core principles of the union - free movement of goods, capital, people and services - are indivisible. That would make points 5 and 8 mutually exclusive.
These contradictions also exist in purely practical terms. For instance it would seem almost impossible to achieve any meaningful control of immigration while still achieving outcomes 3 and 4 on the list. There has been no suggestion of a policy that could bridge this gap.
Such omissions and contradictions are a key reason as to why there is a common belief that no-one inside or outside government has a real working definition of what constitutes a "bad" brexit. It is possible no such definition, and no "red lines" in negotiation actually exist.
This, of course, makes Theresa May's statement that no deal is better than a bad deal yet another example of the confusions and contradictions currently surrounding the process. Indeed, crashing out in the event of a bad deal would in fact seem to contravene goal 12 on her list.