authentication elementmeans an element of a security feature;
covertmeans not directly perceptible by the human senses and detectable only through the use of purpose built tools or professional laboratory equipment.
overtmeans directly perceptible by one or more of the human senses without recourse to external devices.
semi-covertmeans not directly perceptible by the human senses but detectable by those senses through the use of external devices, such as a UV torch or a special pen or marker, which do not require expert knowledge or specialist training.