An element that precedes a given element in a data structure.

A radionuclide that upon disintegration yields a specified nuclide, the daughter, either directly, or indirectly as a later member of a radioactive series.

If an n-electron state is written as a sum of products of 1-electron states and (n - 1)-electron states, the (n - 1)-electron states are called the parents of the n-electron states.