Although Merriam-Webster’s free online dictionary includes the well-known adjective-sense of ‘indeterminate’, it does not include a noun-sense for ‘indeterminate’.
Here are two examples of noun-usage of ‘indeterminate’.
1. “The term “indeterminate” is sometimes used as a synonym for unknown or variable (Becker and Weispfenning 1993, p. 188).”
2. “The idea is to extend the real numbers with an indeterminate i (sometimes called the imaginary unit) taken to satisfy the relation i2 = −1, so that solutions to equations like the preceding one can be found.”
Admittedly, the second example – Wikipedia’s article on complex numbers – is kind of weak, being an inappropriate use of the word, but the usage appears prominently in print, and so it still counts as an example. The problem is that ‘indeterminate’ in this example is actually being used as a weasel word, the correct term to be used being ‘ur-element’ – that is, an arbitrary element whose one and only essential property is that it is NOT a member of the set of extended real numbers.
[errors of omission]