Phantasm (noun)

1. An illusion or mirage, especially one that is seen in the mind.
2. An apparition or ghost.
3. Something that is seen, heard, or otherwise perceived that is not real or present.

Origin:

From latin phantasma, from greek phantasma "apparition, image", from phantazein "to make visible", from phainein "to show".

Examples:

1. He saw a phantasm of his deceased mother in his dream.
2. The phantasm of the castle was caused by the heat waves.
3. The phantasm of the ghostly figure was seen by several witnesses.
4. He had a phantasm of winning the lottery.
5. The phantasm of the unicorn was a popular subject in medieval art.