A specimen was an individual example of its species or type.
In 2254, Captain Christopher Pike was described as a "prime specimen" by Vina because he appeared to be healthy and intelligent. As he matched what Vina's perfect man would be, Pike was picked by the Talosians for one of their experiments. One of the ways the Talosians fed their addiction to mental power was by probing the minds of specimens, such as Pike, and watching them react to and feeling their emotions to illusions. Pike saw himself as a zoo specimen living in a zoo and performing for the zookeepers. In this "zoo", there was a whole collection of specimens whose ancestors had been brought back to Talos IV. The Talosians wished for their specimens to be happy in their lives. (TOS: "The Cage")