A thumbscan was a biometric identification consisting of an image of an individual's thumbprint. The process of obtaining this image was also called a thumbscan. It was often used to record an individual's participation in a contract, presence at an event, or acceptance of an agreement.
In early 2370, a Cardassian Gul's thumbscan on a Kressari PADD proved adequate evidence to show a connection between Cardassia and The Circle — immediately ending that organization and toppling the political career of its leader, Minister Jaro Essa. (DS9: "The Siege")
Later that year, Quark tried to get Inglatu's thumbscan, thus making the contract between the Ferengi and the Dosi official. (DS9: "Rules of Acquisition")