Used as a noun, diplomatic refers to the study of documents and records, their form, language, and script. The term was coined in the seventeenth century and initially embraced paleography and codicology.
Michelle Brown. Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).
Raymond Clemens and Timothy Graham, Introduction to Manuscript Studies (Ithaca, NY: Cornell University Press, 2007), 222-239.