Especially in the philosophy of religion, the term <b>reductio premise</b> is sometimes used.  A Google Scholar search for "reductio premise" (in quotation marks) turns up a few dozen references; one of the most highly cited is <a href="https://web.archive.org/web/20151010180626/http://thorgan.faculty.arizona.edu/sites/thorgan.faculty.arizona.edu/files/Robust%20Vagueness%20and%20the%20Forced-March%20Sorites%20Paradox.pdf">Robust vagueness and the forced-march sorites paradox</a>, by Terence Horgan.

However, among mathematicians, I don't think there is any standard terminology.