Definition: Alpha decay is the spontaneous radioactive decay where an alpha particle is produced.
An atom that undergoes alpha decay will reduce its atomic mass by 4 and become the element two atomic numbers less. The general reaction for alpha decay is
ZXA → Z-4YA-2 + 4He2
where X is the parent atom, Y is the daughter atom, Z is the atomic mass of X, A is the atomic number of X.
Examples: 238U92 decays by alpha decay into 234Th90.