Word for "something that cannot be other than two possibilities" [duplicate]

"Something can either be in state A or in state B; it cannot be in both of these states simultaneously, and it cannot be in neither of them. It has to be in one of those two states". What would those two states be called with respect to each other?

For example, something can either be living or dead, it cannot be both or neither at the same time. So the property of being living or dead would be called _________ of each other. It can be called the opposites of each other but that doesn't convey the full meaning. Because "black" and "white" are "opposites", but something can be other than black and white e.g., red. So it fulfills the second condition of not being in both of the states simultaneously but doesn't suffice the first condition of being in either of those states.

For reference, it is called نقیض(Naqeed) in Arabic. There is a separate word for opposite i.e., ضد (zidd).

Solution 1:

It's a binary choice / decision / classification.

Within which context the two possibilities are mutually exclusive.

Idiomatically, if some issue isn't clear-cut, we might say it's not a black-and-white issue. Most written instances of not a grey area will be for this metaphoric sense - a grey area being to some extent the opposite of a binary choice issue.

Solution 2:

Apparently there does not exist a one word term. A traditional term would be unique alternative.

• In the throwing of a coin the unique alternative to heads is tails. Heads and tails are unique alternatives to one another.

From the SOED

alternative B. 3 Each of the components of an alternative proposition; each of two or more possibilities; the other or remaining course; a thing available in place of another.