Conventional: Something that seems real but is not. A belief about or perception of reality that is false.
ACIM: The Course expands this common definition to include anything outside of Heaven, anything that is imperfect, finite, or painful. This includes any object, situation, or event in the world of time and space, including time and space themselves. It also includes, and mainly refers to, the majority of our thoughts, beliefs, and perceptions while in this world. These are false perceptions of reality, which we dimly recognize as false, yet which we intensely desire and so try to make true (see T-1.IV.2:2, M-8.2:3-4). Truth and illusion have nothing in common; we must choose one or the other. The ego tells us that our only choice is between illusions, deciding which ones we like best. Yet the Holy Spirit teaches that all illusions are equally unreal (see no order of difficulty in miracles) and that all must be brought to truth where they will vanish (see bringing darkness to light). Forgiveness is an illusion, but it is the only illusion that leads to the end of illusions, rather than breeding more. See dream, fantasy.