Definition from Wiktionary, the free dictionary
showstopper (plural showstoppers)
- A performance or segment of a theatrical production that induces a positive audience reaction strong enough to pause the production.
- Any impediment that prevents all further progress; especially a software bug that must be fixed before any further development is possible.
Ambiguous connotation, depending on context: very good or very bad.
(in theatrical production)
impediment to progress