Radioactivity:

Atomic nucleus consists of tightly packed nuclear particles, protons (Z) and neutrons (N) frequently called nucleons, an only exception being the lightest hydrogen nucleus which consists of a single proton only.  Those remain bound within the highly dense nucleus through strong nuclear forces that are operable in a short range of nuclear dimension (nuclear radius ∼ 10^-13 cm or 10^-15 m also known as Fermi).  The sum of protons (Z) and neutrons (N) in a nucleus is referred to as the mass number (A).  All the atoms whose nuclei contain the same number of protons have similar chemical properties.  But atoms of the same element may have a different number of neutrons and, therefore, different A values.  Atoms having the same Z and different A values are called isotopes.  Every isotope has fixed abundance in nature along with few exceptions.  The associative abundance of any isotope (usually expressed as percent abundance) is called the isotopic abundance.  Every element has two categories of isotopes; naturally abundant stable isotopes and unstable or radioactive isotopes that disintegrate spontaneously and eventually become stable isotope of some element.  The radioisotopes decay through emitting elementary particles like as α, β, n, p, β+ or electromagnetic radiations (x or γ-rays) or through undergoing fission (breaking into smaller nuclei).  These isotopes, in a few cases, occur naturally (such as ⁴⁰K, ²D, ¹³C) but are frequently artificial or manmade (such as ⁶⁰Co, ¹³⁷Cs, ⁶⁵Zn, and ²³⁹Pu).

That is customary to call the decaying nuclide as 'Parent' and the resultant nuclide as 'Daughter' that in turn might again become parent in case of subsequent decay.