This tag stands at over 100 posts and lacks a wiki entry, but I'm unsure of how to define it. Many, if not most, of the images tagged lampion have what we would ordinarily tag as paper lantern, which would the tag redundant. Frustratingly, Google images turns up exactly the same results, leaving me to wonder what to use this tag for.

All the definitions I can find for "lampion" describe an entirely different sort of object, typically oil-burning lanterns made with glass, but sadly no pictures accompany these definitions. The closest thing I can imagine is lanterns like those in post #1143160 and post #2174052, but I have no idea how similar those objects are to historical real-world lampions or whether it would be appropriate to tag them as such.

Regardless of how the tag is to be used, there's no reason I can think of for continuing to use it interchangeably with paper lantern. Two tags shouldn't mean the same thing.

Doesn't it refer to a paper lamp that can be collapsed? The ones on the Google Image search all have a corrugation as if they're meant to be collapsed like an accordion. Hence Lamp + Accordion = Lampion?

The Japanese term for what iridescent_slime calls lampions is 豆ランプ. Try this google image search.
It is not used as a pixiv tag, though.

@Ai-to-Yukai
The Google search result you see may be different from mine, then, because I see a lot of sky lanterns and other lanterns made from un-corrugated paper. For example. The "accordion" idea is clever but appears to be a false etymology.

@henmere
Those lamps seem to fit wiktionary's definition in English as "a small oil lamp". I wonder if it would be better to merge images of lamps like those into the kerosene lamp tag, though.

Merriam-Webster Online Dictionary:

lampion. n. plural -s. archaic: a small lamp (as a pot of oil with a wick) formerly used at illuminations.

Oxford Online English dictionaries don't list the word. Wikipedia redirects the word to paper lantern.

Given Merriam-Webster's definition, I'd take it it refers to the burner itself in these paper lanterns, and then people just started referring to the paper lantern itself by the term.