It’s an interesting topic. I think the traditional distinction is, as you said, the purpose for which the coin was produced. As with all such distinctions, it’s easiest to recognize them at their extremes, but the fuzzy boundaries are where much of the fun is to be found.
In the ideal version, medallions would be “ceremonial,” marking a specific event or person, and dispersed ritually to important people. "Coins" -- even big & beautiful coins -- are ideally meant to be spent and fit well into known
denominations.
To me, things get interesting where the categories overlap and/or when a thing that started out as one becomes the other. Such as the Alexandrian Drachms that were sometimes pierced to be used as "
amulets" (possibly "funerary
amulets"), one of mine in this comment:
https://www.forumancientcoins.com/board/index.php?topic=130587.msg780981#msg780981 I like the
German term, “Großbronze” (large bronze). I think it can be used without taking a hard stance on whether something is a
medallion or coin. I’ve got a few extra-large
Roman Provincial Großbronzes from
Cilicia that seem to fit.
I've just added the next two to my
gallery, since I
had photos ready. First one is 35-36mm, about 34g (sometimes called a "Hexassarion," this one is heavier than at least 27 of 28 specimens on
RPC, the 28th a modified ex-jewelry coin):
Gallery:
https://www.forumancientcoins.com/gallery/displayimage.php?pid=181200Another interesting one may be
part of
Antoninus Pius’ series of similar AE Tetrassaria (?) at
Hadrianopolis (
Thrace), though on an oversized
flan (34mm, 21.25g). (Only example of this
reverse type, so the comparison specimens are the other Antoninus
AEs from
Thrace.) It has the “look” and
diameter of an AE
Medallion, but not the
weight.
Gallery:
https://www.forumancientcoins.com/gallery/displayimage.php?pid=181201 (see also:
https://www.forumancientcoins.com/board/index.php?topic=93598.msg779567#msg779567).
So, while I have some
medallic coins, I’m not sure I’ve ever owned any actual ancient
medallions.
I also like the small fractional medallions, sometimes called “medalettes.” (The big AV/AR medallions that get all the attention are multiples of
standard denominations, though surely not intended as “
money”; e.g., an AV Denio or “
Medallion of Ten
Aurei”
[LINK] or “
Medallion of 10
Denarii”
[LINK]).
For the smaller ones, though, it seems like there’s often debate about whether they’re actually coins, tokens/tesserae, or medallions of some sort. I’m thinking, for example, of certain Quadrans-sized “medalettes” from 1st or 2nd century, and some of the 4th cent.
Constantinople commemorative series. (I like the
anepigraphic so-called AR
Half-Siliqua Medalettes:
https://www.acsearch.info/search.html?id=287833.) Incidentally, Leu just auctioned a series of them from the
collection of Lars Ramskold (
https://independent.academia.edu/LarsRamskold), who published a series of interesting papers on medallions and
Constantinopolis commemorative coinage. Leu WA 25, 2572-3263 [numisbids
LINK]