Primordial nuclides are nuclides found on Earth which have existed since before the Earth was formed. They are also called primordial isotopes.
Primordial nuclides are found in the interstellar medium. They were formed in or after the Big Bang, by nucleosynthesis in stars and supernovae. or by being hit by cosmic rays. Only 286 primordial nuclides are known.
There are 252 stable nuclides, plus another 34 nuclides that have half-lives long enough to have survived from the formation of the Earth. These are the primordial nuclides. The 34 primordial radionuclides are isotopes of 28 separate elements.