A nuclide is defined conventionally as an experimentally examined bound collection of protons and neutrons that either is stable or has an observed decay mode.
In this latter table, where a decay has been predicted theoretically but never observed experimentally (either directly or through finding an excess of the daughter), the theoretical decay mode is given in parentheses and have "> number" in the half-life column to show the lower limit for the half-life based on experimental observation.
These have been measured to be radioactive, or decay products have been identified in natural samples (tellurium-128, barium-130).
With most of these 25, decay is difficult to observe and for most purposes they can be regarded as effectively stable.
Bismuth-209 is notable as it is the only naturally occurring isotope of an element which was long considered stable.
They are the primary source of radiogenic heating and radioactive decay products.
These include ~44 radionuclides occurring in the decay chains of primordial uranium and thorium (radiogenic nuclides), such as radon-222.
Others are the products of interactions with energetic cosmic-rays (e.g. cosmic ray spallation) (cosmogenic nuclides), such as carbon-14.
Further shorter-lived nuclides have been detected in the spectra of stars, such as isotopes of technetium, promethium, and some actinides.