Essential oil content, more precisely called volatile oil content, is the aromatic fraction of a herb or spice that can be distilled off with steam. It is determined by hydrodistillation to ISO 6571 and reported in millilitres per 100 g on a dry basis. It is the closest thing the category has to a single measure of flavour value.
Why it works this way
Every other parameter on a herb specification describes what should not be there: moisture, ash, sand, microbial load. Volatile oil is the only one that describes what the buyer is buying. Two lots of dried oregano can pass an identical specification on every other line and deliver noticeably different flavour, because nothing in that specification measured the aroma.
The dry-basis qualifier matters. Volatile oil is reported per 100 g of dry matter, so a wetter lot would otherwise report a lower figure for no quality reason. That is why the moisture figure has to appear on the same certificate from the same sample.
The reference document that most European buyers name in a contract is the European Spice Association Quality Minima Document, which sets a minimum volatile oil alongside maximum moisture and maximum ash for the main commodities. It is a trade reference rather than legislation, and the revision in force at contract date governs.
What follows in practice
Write four things into the clause: the method (ISO 6571), the basis (millilitres per 100 g dry), the accompanying moisture requirement, and the point of measurement. Then compare offers on volatile oil per kilogram rather than on price per kilogram, because a cheaper lot at a lower oil content is often the more expensive one in the finished product.
Herb drying and essential oil retention explains where the oil is lost and how much of it a given process keeps.