Tertium non datur - Latin expression. Literally translates third is not given. But since Latin loves idioms as much as English, a better translation would be non-literal, i.e., there is no third option, or there is no alternative, or, by the comparable English idiom, take it or leave it.

The expression can be used in contract "negotiations": "Here's your contract, sign it as is or don't sign it at all."

It can also describe any kind of dilemma, when we seem to have to choose from two options, especially when we don't like either.

Further, it can express the Aristotelian attitude which has been shaping the Western mind for millennia: You can be good or evil, saint or sinner, friend or foe, strong or weak, smart or stupid, success or failure, right or wrong, after you die you either live forever or cease to exist altogether (i.e. eternalism vs. nihilism), etc, etc, etc.

Let me propose that tertium non datur is always a fallacy: Non tantum tertium sed quartum et quintum. ("Not only third, but fourth and fifth.") There always are many alternatives. We only fail to see them, or perhaps don't want to see them. But they always exist. And finding them is the only way to go.