What is the meaning of Decay?

Rot; any processes or result of organic matter being gradually decomposed, especially by microbial action.

Deterioration of condition; loss of status, quality, strength, or fortune.

Radioactive decay; decomposition of an atom or its nucleus.

Particle decay; decomposition of a sub-atomic particle.

Continuous decrease of a quantity.

Overthrow, destruction, ruin, death.

Array decay.

To deteriorate, to get worse, to lose strength or health, to decline in quality.

To undergo bit rot, that is, gradual degradation.

To undergo software rot, that is, to fail to be updated in a changing environment, so as to eventually become legacy or obsolete.

To undergo prolonged reduction in altitude (above the orbited body).

To rot, to go bad.

To change by undergoing fission, by emitting radiation, or by capturing or losing one or more electrons; to undergo radioactive decay.

To undergo optical decay, that is, to relax to a less excited state, usually by emitting a photon or phonon.

Loss of airspeed due to drag.

To cause to rot or deteriorate.

Of an array: to lose its type and dimensions and be reduced to a pointer, for example when passed to a function.

Source: wiktionary.org