Nova refers to optimizing output in a single turn. It is taking your best shot. The best Nova round I've seen in 5e was in a one shot adventure for 20th level PCs where folks were told to make the mst efficient PC they could. They were able to have two legendary items and up to 5 other magic items. The Nova king was a paladin 11, warlock 2, fighter 2. On his turn ge was able to attack twice, action surge to attack twice more, bonus action attack, reaction attack, and haste attack. All of those were GWM attacks, all included sites using advanced spell slots, all were with a weapon dealing an extra 2d6, all with Str 29, etc... 28d6 + 42d8 + 113... 400+ damage IIRC. He soloed one of the ancient dragons by himself in one round. I also saw a variant with a paladin 2, eldritch knight 17, warlock 1 with slightly less impressive Nova damage, but he was able to Nova twice and split the damage out into more attacks to use it more efficiently.
The only Nova issue I have encountered in 5e is the one from that situation: Multiple smites on multiple attacks.