Why does deuterium have the name heavy hydrogen?

Deuterium is referred to as "heavy hydrogen" because its nucleus contains one proton and one neutron, giving it approximately double the mass of the nucleus of ordinary hydrogen, which consists of only a single proton.12

This increased mass is also reflected in compounds formed with the isotope. For example, heavy water ($D_{2}O$) is composed of two deuterium atoms and one oxygen atom, resulting in a higher molecular weight than ordinary water ($H_{2}O$). Because of this significant mass difference, deuterium and its compounds exhibit distinct physical properties compared to their ordinary hydrogen counterparts, such as differences in boiling points and vapour pressures.31