noun which may take the genitive case particle 'no'
imaginary; in one's imagination
Used to describe something that exists only in the mind, not in reality. Often modifies nouns with の.
それは想像上の生き物だ。
That is an imaginary creature.
彼は想像上の友達と話していた。
He was talking to an imaginary friend.
仮想 means 'virtual' or 'suppositional', often used in computing or hypothetical scenarios. 想像上 is more about mental imagery.
Compound of 想像 (imagination) + 上 (on, above), literally 'on imagination', meaning 'in the realm of imagination'.