What do you call this fraction?

I would say "one over twenty-one," or "one divided by twenty-one." For more information look at this website.

Edit As Jim has pointed out in the comments, "one twenty-first" and "five twenty-firsts" are also correct pronunciations, although they might be a little awkward to say.

You will find that dictionaries like this one use the ordinal to define the fraction:

being one of 21 equal parts into which something is divisible <a twenty-first share of the money>

The courts in New Jersey have no trouble using the twenty-first part of estates divided per stirpes:

Therefore, upon the death of Frank, one-seventh equals three-twenty-firsts passed to his son. Anna, dying childless, her two-sevenths equals six-twenty-firsts passed to Mary , Henry, and William equally; two-twenty-firsts to each.

Wiktionary states that oneth - and, I guess, beyond - is a

Noun

(in compounds with twenty-, thirty-, forty-, etc.) A fractional part of an integer ending in one.

(in algebraic expressions) An ordinal value that is represented by an expression ending in 1 such as the (n + 1)th.

In mathematical expressions such as (n + m), where these are variables, for (cardinal) numbers, say, in equations, is there an ordinal expression for m?

As well, what if there are numbers "below" zero, as there may be numbers "above" the reals of infinity, its levels, and, then, the hyperreals, and surreals? I mean, what if zero is a number, ie, the first (integer) number? Then we run into stuff like calling the 1800's the nineteenth century. Perhaps, it's best to put n equal to zero, with m equal to the repeating decimal fraction 0.238095 . Then it, too, is just another number. And, we see what the number is made of.