-10x^8 is indeed a monomial, because there is only one term. There is nothing else added or subtracted.
There are three types of "nomials":
Monomials: such as the one you have, they only have one term
Example: y, 6x^2, 19084v^22341
Binomials: Equations with two terms
Example: x+1, v+4w, k+1021s
Polynomials: Equation with more than three terms (no limit)
Example: x+5+9z, t+9u+8v
