Which word best describes something that is not important?

The word "trivial" is an appropriate choice to describe something that is not important. It indicates that the subject or matter being referred to has little value, significance, or relevance in a given context. Trivial matters are often deemed inconsequential, suggesting that they do not warrant attention or concern in contrast to matters that hold genuine importance or weight.

In contrast, other terms such as "significant," "crucial," and "essential" denote levels of importance and necessity, which is the opposite of trivial. "Significant" implies that something has meaning or impact; "crucial" indicates that it is vital or critically important; and "essential" suggests something is necessary or fundamental. Therefore, "trivial" distinctly conveys a lack of importance, making it the best choice for this question.