What word means unpredictable or eccentric in behavior?

The term that means unpredictable or eccentric in behavior is "erratic." This word is derived from the Latin "erraticus," meaning "to wander," which captures the essence of being inconsistent or deviating from the norm. When someone exhibits erratic behavior, their actions can vary significantly, often making it difficult to anticipate how they will respond in different situations.

In contrast, the other choices suggest different qualities: "stable" describes a constant and reliable state, "unstable" implies a lack of stability but does not convey the eccentricity or unpredictability of actions, and "methodical" indicates a systematic and orderly approach, which is the opposite of being erratic. Therefore, "erratic" is the only term that accurately reflects the idea of unpredictability and eccentric behavior.