Which word describes something that is irregular or unpredictable?

The term "erratic" specifically denotes behavior or phenomena that lacks consistency or regularity, making it inherently unpredictable. When something is described as erratic, it suggests that it deviates significantly from what is considered normal or expected, displaying patterns that are uncertain and often turbulent. This term is commonly used in various contexts, such as weather patterns, financial markets, and even in describing someone's mood or performance, where the fluctuations can be quite significant and hard to anticipate.

The other choices do not convey the notion of irregularity or unpredictability. "Equilibrium" refers to a state of balance or stability, suggesting predictability rather than erratic behavior. "Exhaustive" describes something that is thorough and comprehensive, while "engrossing" pertains to something that captures attention fully, neither of which relate to the concept of being unpredictable. Thus, "erratic" is the most suitable choice to embody irregularity or unpredictability.