What is a type specimen?
A type specimen is a single plant specimen chosen by the botanist who first named and published the species to represent the species as a whole. That specimen then becomes the definitive example of that species. This specimen (as a dried plant or occasionally an illustration) is preserved in a herbarium It can then be consulted when similar species are found or related species need to be identified.
The definition given above is strictly for what is termed a "holotype". There are other categories of type specimens. The NYBG Steere Herbarium website defines these other categories if you are interested:
Courtesy of NYBG Plant Information Service