Sadhu Bhante, so well said ! 
In Meditation Viveka comes, “Vivekanupassi viharathi, Niridhanupassi Viharathi” What does this mean? Niroda is when it is all stopped meaning that no longer is there ability of Nama-Rupa as the consciousness is now non-illustrated. Viveka thus mean where it is the point when the illustrations of nama-rupa are no more, leading to Nirodha. So Viveka is not a thing like being alone or seperate.
If I am to add further ti may earlier reply, “Viveka” means free of all defilings. “Viveka nissithan, viraga nissithan, nirodha nissithan, vossagga parinama”, free of all defilement, cleansed of desire, no more birth, Nirvana.
Bhante ,
nimitta , your translation here appears not correspond to what normally refer to as an aspect or sign ?
「Whatever is one-pointedness of mind, friend Visākha, that is concentration, the four ways of attending to mindfulness are the causes of concentration, 」
Bhikkhu Ānandajoti
「And what, lady, is concentration, what are the distinguishing marks of concentration 」
I.B. Horner