As you begin to research using security printing to secure your product or documents, you will hear a lot of terms being tossed around.
Two terms commonly used are Covert and Overt. Similar words with opposite meanings.
Covert simply means a hidden image invisible to the naked eye. An example of a covert image would be one printed with invisible UV inks that fluoresce under UV light. The example in this picture is a €20 banknote under ultraviolet light