Isotopes are the elements having same atomic no. But different mass no. Like protium and deuterium

ISOTOPE:Two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties, a radioactive form of an element.Carbon 12 the most common form of carbon has six protons and six neutrons whereas carbon 14 has six protons and eight neutrons. Isotopes of a given element typically behave alike chemically.

For example hydrogen
The atoms of same elements which have same atomic number but different mass number are called isotopes .