a figment of someone's imagination

US /ə ˈfɪɡmənt əv ˈsʌmwʌnz ɪˌmædʒɪˈneɪʃən/
UK /ə ˈfɪɡmənt əv ˈsʌmwʌnz ɪˌmædʒɪˈneɪʃən/
"a figment of someone's imagination" picture
1.

某人想象的产物, 凭空想象

something that someone has imagined and that does not really exist

:
The monster under the bed was just a figment of his imagination.
床下的怪物只是他想象的产物
Her fear of flying is not a figment of her imagination; it's a real phobia.
她对飞行的恐惧并非凭空想象;那是一种真实的恐惧症。