Radioactive decay or atomic decay

Dental caries, also known as tooth decay

Bacterial decay, decomposition of organic matter

Half-Life: Decay, a computer game add-on

When a sound reverberates in a room, "decay" means the sound fading away

Particle decay, in physics

Orbital decay, the process of prolonged reduction in the height of a satellite's orbit.

Decay (DC Comics), a comic book character