What term defines something that cannot be copied or imitated?

The term that defines something that cannot be copied or imitated is "inimitable." This word suggests a quality of uniqueness or distinctiveness that sets something apart from others, making it difficult or impossible to replicate. For example, a piece of art that is described as inimitable possesses a style, technique, or emotional depth that cannot be reproduced, even by skilled imitators.

In contrast, the other terms have different meanings. "Limpid" refers to something that is clear or transparent, often used to describe water or a person’s expression. "Ineluctable" means unavoidable or inescapable, typically pertaining to situations or outcomes that cannot be avoided. "Lassitude" indicates a state of physical or mental weariness; it embodies a sense of fatigue or a lack of energy. Thus, while these words have distinct definitions and usages, "inimitable" specifically captures the idea of something that stands alone in its uniqueness, making it the correct choice.