1.
illusion, phantom, vision, apparition
Something that appears to exist but does not, or that existed only briefly and is now gone. Used for visual hallucinations, fleeting experiences, and things that seem dreamlike or unreal.
2.
legendary, elusive, rare beyond belief
Used as a modifier (幻の) to describe something so rare or fleeting that it seems almost mythical or impossible to obtain.
Usage
幻 as a standalone noun means an illusion or vision — something that appears real but is not. As a modifier (幻の), it describes something so rare, elusive, or short-lived that it takes on a mythical quality. This modifier usage is very common in food and drink writing (幻の日本酒, 幻の食材) and in media (幻の名作).