“难下定义的事物”可以翻译为 "things that are difficult to define" 或 "elusive concepts/entities"。
"Things that are difficult to define" 直截了当地表达了难以给出明确定义的事物这一含义。
"Elusive concepts/entities" 中，“elusive”意为“难以捉摸的；不易理解的”，“concepts”指“概念”，“entities”指“实体”，这种表达更侧重于描述事物本身的模糊性和难以界定性。