An object is prompt if it originates directly from something, usually a primary vertex of the event, as opposed to some subsequent decay of e.g. a hadron or tau lepton or other particle that is not a heavy resonance of interest. The exact meaning of promptness depends on the context.
Definitions in use
Rivet link to Rivet source Promptness is undone by being directly or indirectly color-connected to other parts of the event.