What term refers to something that is essential or fundamental to a process?

The term that most accurately refers to something that is essential or fundamental to a process is "integral." This word conveys the idea of something being necessary for the completeness or functioning of a system. When something is described as integral, it is implied that without it, the process cannot achieve its intended outcome, much like how integral parts of a machine work together to ensure the machine operates effectively.

While "core," "primary," and "major" also suggest importance, they may not capture the specific notion of being indispensable within a process. "Core" often refers to the central or most important part, which is important but does not fully encompass the idea of necessity in the same way. "Primary" implies first or most important in rank or order but does not necessarily indicate that it is essential for a process to function. "Major" suggests significance or importance but lacks the depth of necessity that "integral" encapsulates.

Therefore, "integral" stands out as the most precise term for describing something fundamental to a process.