He thought he saw an oasis in the desert, but it was just a hallucination.

hallucination 是由动词 hallucinate（产生幻觉）加上名词后缀 -ion 构成的名词，表示"幻觉、错觉"。这个词来自拉丁语，原意是"在脑海中游荡的想法"。