There are unequivocal industry standards to define what can be called a EVOO and what cannot be called EVOO and you can read about them here. Having said that, quality is not defined by these criteria, they are an industry standard agreed amongst producers and bottlers with very different and often conflicting objectives, thus they are a compromise that helps categorize minimum requirements for EVOO labelling.
A great EVO oil must by all means have certain chemical characteristics, amongst them a low content of free fatty acids below 0.025, low peroxides below 5, low acidity below 0.5, high Phenol count above 400 ppm and high content of Tocopherols over 160 ppm. These key chemical ingredients are not enough in defining quality, they are just part of the story.
Today is a period of great awareness of the biochemical processes of olive production and oil extraction; it is the first time in thousands of years that so many marvellous options are available to farmers and oil producers to influence and steward their work towards specific quality goals.
This opportunity allows producers to follow different passions, paths and intuitions. Every farm, every mill and thus each EVOO is ultimately a story made by and of human beings, their emotions, their dreams, tastes and by their ethics.