Decay may refer to the following:
Bacterial decay, decomposition of organic matter
When a sound reverberate in a room, "decay" means the sound fading away
Radioactive decay or atomic decay
Dental caries, also known as tooth decay
Decay , a comic book character
Decay, a computer game add-on
Particle decay, in physics
Orbital decay, the process of prolonged reduction in the height of a satellite's orbit.