Which term describes something that is difficult or impossible to exist without?

The term that most accurately describes something that is difficult or impossible to exist without is "indispensable." This word conveys the idea of something being absolutely necessary or essential to the point that its absence would hinder the function or existence of a particular thing or system. In many contexts, such as in discussions about resources or relationships, the use of "indispensable" highlights the critical role of the element in question.

While the term "essential" also implies a high level of necessity, it can sometimes be interpreted as slightly less stringent than "indispensable." "Optional" implies that something is not necessary and can be disregarded, while "superfluous" indicates that something is unnecessary or excessive. This distinction clarifies why "indispensable" is the strongest choice in describing an element that one cannot do without.