The term used to describe the difference in number of neutrons in isotopes is known as:

The term that accurately describes the difference in the number of neutrons among isotopes is known as the mass number. The mass number is the total count of protons and neutrons in an atomic nucleus. Since isotopes of the same element have the same number of protons but differ in their neutron count, this difference in neutrons directly affects the mass number of each isotope.

For instance, carbon-12 has six protons and six neutrons, giving it a mass number of 12, while carbon-14 has six protons and eight neutrons, resulting in a mass number of 14. Thus, the mass number reflects the variations in neutron counts between isotopes of a given element.

The other options, while relevant to atomic structure, do not specifically indicate the differences in neutron count characteristic of isotopes. For example, nuclear charge refers to the total positive charge of the nucleus due to protons. The neutron ratio would imply a comparison or relationship between numbers of neutrons rather than defining the concept, and the atomic number denotes the number of protons in the nucleus, which remains constant across isotopes of a given element.