Definitions for illusiveɪˈlu sɪv
Random House Webster's College Dictionary
Origin of illusive:
based on or having the nature of an illusion
"illusive hopes of finding a better job"; "Secret activities offer presidents the alluring but often illusory promise that they can achieve foreign policy goals without the bothersome debate and open decision that are staples of democracy"
Subject to or pertaining to an illusion, often used in the sense of an unrealistic expectation or an unreachable goal or outcome.
Testing software completely is an illusive goal.
deceiving by false show; deceitful; deceptive; false; illusory; unreal