Final answer:
The polynomial −625a^8 + 16b^4 is a binomial since it is composed of two unique terms.
Step-by-step explanation:
The polynomial −625a^8 + 16b^4 consists of two terms and therefore is classified as a binomial. A binomial is a type of polynomial with exactly two terms, unlike a trinomial which has three terms, a quadrinomial which has four, or a monomial which has just one. This classification is based on the number of distinct terms in the expression.