illusive

Origin:

early 17th century: from medieval Latin illusivus, from Latin illus- 'mocked', from the verb illudere (see illusion)

Do not confuse elusive with illusive. Elusive means 'hard to find, catch, or achieve' (what I was looking for proved elusive), whereas illusive means 'not real, although seeming to be' (the illusive nature of the material world).