Sorry for not being clear anough: I didn’t want to suggest a new syntax for definition lists yet. Just the general thought, that description lists (as they are newly called in HTML5 instead of “definition lists”) can have a different and more frequent usage than just for a glossary or an alphabetically sorted technical terms list.
That means that I see the “stage I°” as being a list symbol just like “1.”, “a)”, “i.” or bullets and dashes.
I have seen proposals for an inline definition list syntax in this forum. I think someone suggested ~: or :~ (can’t remember exactly). Maybe this is indeed a good solution.
Other option i would make up, that is indeed similar:
~ term1 : definition 1
~ term2
: definition 2
The tilde is indeed similar to the dash in visually representing a new list item, but there would be no ambiguity about whether you want an unordered list item or a description list item.
The tilde would also remove the need for a look-back when parsing an ddetecting a defintion list.
The combination would probably make the whole thing quite distinct. Both the tilde and the colon could still be easily used for other purposes, because they would only be a description list in combination. To get a empty description use ~ term : to get an empty term use ~ : description.
I would keep the space before the colon _:_, because this would make escaping needed less often in cases like ~ cell cycle: anaphase : description, but possibly the dense version would be no problem either.