During his severe depression, he found it difficult to concentrate on anything.

depression 由前缀 de-（表示"向下"）和词根 press（压）以及名词后缀 -ion 组成，字面意思是"向下压的状态"。在心理学中，指持续的情绪低落状态，即"抑郁症"。severe depression 特指症状严重的抑郁状态。