The half-life of a radioactive nuclide is defined as the time it takes half of a sample of the element to decay.
An alpha particle contains two protons and two neutrons.
Protons and neutrons together are called nucleons, meaning particles that can appear in the atomic nucleus.
A nuclide of an element, also called an isotope of an element, is an atom of that element that has a specific number of nucleons.
The decay rate and therefore the half-life are fixed characteristics of a nuclide. Thats the first axiom of radiometric dating techniques: the half-life of a given nuclide is a constant.
(Note that this doesnt mean the half-life of an element is a constant.