A category that is constructed as being the ultimate out-group, the category that an entire society defines itself against.
From the thesis, page 47:
"The opposite of zero-category may be called abyss-category. As norms are defined by their deviations and these deviations are conceptualized as categories of people, society itself may point to certain categories as being the ultimate deviation for everyone to define themselves against. Such an “abysscategory” of contemporary mainstream western society can be nazis, fascists or pedophiles – thus the concept of “Godwin's Law”, the sarcastic remark that "As an online discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches 1."98 In a more religious society, the abyss-category may instead be atheists, witches, or anyone who believes in a slightly different version of the mainstream theology. For example with Protestants and Catholics considering each other to be the spawn of the devil, or with Sunni and Shia Muslims feeling the same way about each other."