Any idea to what $\bar{X}$ refers to in association rules? Is it "not-$X$"?
Do you have a link to the actual source you are referring to? I haven't myself seen it before in the context of association rules and interestingness.



As was pointed out by @Sean Owen in his comment, it is one of a few standard notations for saying something is not in a set - i.e. the complement. This would make most sense, as we are generally referring to itemsets when computing the associations.

Regarding the complement and set notation, the Wikipedia article contains this:


  The absolute complement of A is usually denoted by $${\displaystyle  A^{\complement }}$$ 
  
  Other notations include $$ {\displaystyle A^{\text{c}}}, \hspace{3mm} \overline A, \hspace{3mm} A',\hspace{3mm} \complement _{U}A \hspace{3mm} and \hspace{3mm} \complement A$$


Of course a bar can also mean things such as the mean or expectation value of some values (I have even seen people using it to mean the unobserved or predicted value!), but with the information you provide, the complement seems most likely.

Here also from the same Wiki page linked above for those who are unsure, a visual definition of the complement: