How close the speakers are at portraying the live event, whether it be a chamber ensemble, symphony orchestra, intimate jazz performance, or an outdoor rock concert. No speaker that I've run into can replicate that spectrum perfectly - there are always compromises with any design choice.
Only two speakers I've encountered that got close to ONE of those aforementioned mile markers:
Wilson WATT/Puppy 5s reproducing Horowitz on stage
Dunlavy SC-IV/As playing an acoustic performance of "Hotel California".
That said - the Wilson wasn't able to "do live" as well as the Dunlavy; and that speaker had a hard time not getting "gritty" with solo instrument performances.