I believe it helps graceful fallback, hence backwards compatibility, to throw a colon and a space in there. That pseudo URI is really unobtrusive.
[abbr](: "expansion")
My preferred unified abbreviation and term definition syntax, however, includes straight apostrophe or quotation marks and round parentheses:
'expansion' (abbr)
'definition' (optional text: "term")
(Details, like number and type of apostrophes, are of course open to improvement. The space before opening parenthesis may need to be suppressed.)