Glossary

Atom
Is the smallest unit that defines the chemical elements and their Isotopes.

Half-life
Is the amount of time required for a quantity to fall to half its value as measured at the beginning of the time period.

Ion
Is an atom where the number of electrons is not equal to the number of protons.

Isotope
Is a variant of a particular chemical element such that while all Isotopes of a given element have the same number of protons, but they differ in neutron number.
Nucleus
Is the very dense region consisting of protons and neutrons at the centre of an atom.

Nuclear
Is a branch of physics concerned with the understanding of the atomic nucleus, including its composition and the forces which bind it together. 

Radioactive decay
Is also known as nuclear decay or radioactivity and is the process by which the nucleus of an unstable atom loses energy by emitting ionising radiation. 

Radioactive isotope 
Is an atom with an unstable nucleus, characterized by excess energy available to be imparted either to a newly created radiation particle within the nucleus or via internal conversion. During this process, the radionuclide is said to undergo radioactive decay, resulting in the emission of gamma ray(s) and/or subatomic particles such as alpha or beta particles. These emissions constitute ionising radiation. Radioactive isotopes occur naturally, or can be produced artificially. 

Stable isotope
Is an atomic species that is not radioactive and does not undergo radioactive decay. 

NOTE:
The content of this glossary is taken from Wikipedia and provides a basic explanation only, is by no means complete and may be extended.