Which word describes something that is unpredictable and eccentric?

The word that best describes something unpredictable and eccentric is "erratic." This term is often used to characterize behavior, patterns, or occurrences that are inconsistent, varying greatly, or lacking a fixed course. When something is described as erratic, it suggests a level of unpredictability; for example, an erratic driver may change lanes unpredictably, just as an erratic weather pattern might shift unexpectedly, leading to sudden changes in conditions.

In contrast, the other options present characteristics that are stable and predictable. "Routine" refers to actions or events that occur in a regular, methodical way, emphasizing consistency. "Commonplace" signifies that something is typical or ordinary, lacking uniqueness or eccentricity. "Dependable" characterizes something or someone as reliable and consistent, which is the opposite of unpredictability. The essence of "erratic" captures the quality of being unusual and not following a norm, making it the most fitting choice for the given definition.