Physics term - covers both nuclear decay (α, β, γ types) and particle decay generally. The crucial compound is 半衰期 (bànshuāiqī, "half-decay-period") = half-life, fundamental in nuclear physics, geology (carbon dating), and pharmacology (drug clearance times).
memory
衰 (decline) + 变 (change) - the change that's a decline. Don't confuse the verb sense with general 衰退 (shuāituì, "decline" of an economy) - 衰变 is strictly the physics meaning.
