A figment of somebody's imagination 的意思是“一个人凭空想象的事物”。它着重强调虽然一个人觉得某事物是真实的，但其在现实中并不存在，仅仅是臆造的东西。它可用于描述人、人的思想和事物。

例句

A: Do you think Mary is interested in me?

A: 你觉得玛莉是不是对我有意思？

B: No, that's a figment of your imagination. She's happily married with two kids.

B: 不可能，你自己瞎想什么。她婚姻幸福，还有两个孩子。

Was it a figment of my imagination or did I just see John driving away on my beloved motorbike?

是我凭空想象出来的还是刚才我真的看到约翰骑走了我最心爱的摩托车？

Is Santa real or just a figment of people's imagination?

圣诞老人是真的，还是人们凭空臆造出来的？