Which term describes something that is not possible or imaginable?

The term "implausible" refers to something that is not believable or likely to happen; it suggests that a situation, event, or idea lacks credibility or acceptability in terms of its realism. When describing something as implausible, you are indicating that it stretches the boundaries of what one could reasonably conceive as possible or probable.

In the context of the question, "implausible" effectively captures the essence of something being not only unlikely but also difficult to imagine occurring without significant disbelief or skepticism. This makes it the most suitable choice for a term that describes concepts that are beyond the realm of possibility or imagination.

The other terms, although related, do not convey the same level of impossibility. "Improbable" indicates something that is unlikely but still possible. "Impractical" refers to something that is not sensible or feasible in terms of execution or application, which does not directly imply that it is unimaginable. "Indefinable" suggests something that cannot be clearly defined or categorized, but it does not necessarily speak to the concept of possibility or imagination. Therefore, "implausible" is the most fitting answer for describing something that is not possible or imaginable.