EN 166 is a particular safety norm. When a piece of eyewear is EN166 certified it generally means that it is a piece of safety eyewear.
The correct term is actually EN 166 personal Eye Protection European Standard, but is normally referred to as EN166 or EN 166. It is the core technical industrial safety norm in Europe for eye protection applying to all types of individual protection of the eye which protects from hazards likely to damage the eye, except for nuclear radiation, x-rays, laser emissions and infrared emitted by low temperature sources.
EN 166 is often the minimum required certification regarding eyewear in Personal Protective Equipment (PPE).