The fallacy of suppressed correlative is a type of argument that tries to redefine a correlative (one of two mutually exclusive options) so that one alternative encompasses the other, i.e. making one
A conceptual example:
Person 1: "All things are either X or not X." (The correlatives: X–not X.)
Person 2: "I define X such that all things that you claim are not X are included in X."
(The suppressed correlative: not X.)
Alternatively Person 2 can redefine X in way that instead concludes all things are not X.
A simple example based on one by Alexander Bain.
Person 1: "Things are either mysterious or not mysterious. Exactly when an earthquake will strike is still a mystery, but how blood circulates in the body is not."
Person 2: "Everything is mysterious. There are still things to be learned about how blood circulates."
There is more by way of explanation, follow the link. RK