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.

某人想像的產物, 憑空想像

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像的產物
Her fear of flying is not a figment of her imagination; it's a real phobia.
她對飛行的恐懼並非憑空想像;那是一種真實的恐懼症。