German term (edited): differentia spezifica

Given that this is aimed at an academic readership, I think it would be appropriate to use the Latin term. Note that "differentia" is singular (the plural being differentiae). The term is used primarily in logic, though there is some overlap between logic and linguistics. The differentia of something is that property which distinguishes it from others.

"By making men into men, Prometheus’ subversive act becomes the symbol for men’s becoming political beings; and fire, in turn, becomes the symbol less of culture, reason, science or art, than of the primal fabric of men’s political essence... Fire becomes, to borrow Blumenberg’s phrase (1990, p. 308), men’s **‘differentia specifica’**."
http://www.psa.ac.uk/journals/pdf/5/2003/Dimitrios Akrivouli...

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Note added at 52 mins (2007-08-25 18:56:15 GMT)
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"In the twentieth century, the view that the **differentia specifica** of poetry can be located in formal characteristics, such as metre, formulas, or poetic language, was firmly put forward by structuralist critics, especially the Prague School. For example, Roman Jakobson in a famous article defined the poetic function of language as directedness toward the "message", that is, the verbal sign itself (1960: esp. 353-6), and went on to discuss linguistic criteria for poetry."
http://fds.oup.com/www.oup.co.uk/pdf/0-19-924550-9.pdf