The word nonlinear refers to a system or process that does not follow a straight or predictable course. Some synonyms for nonlinear include dynamic, unpredictable, complex, irregular, and erratic. These words indicate the lack of linearity and instead describe a system that can change rapidly, have multiple outcomes, or be impacted by a range of factors. Other synonyms for nonlinear could include non-sequential, branching, divergent, or non-cyclical. Each of these words contributes to the overall understanding of a nonlinear model or process and helps to convey the complexity or unpredictability of the system in question.