decay

from The American Heritage® Dictionary of the English Language, 5th Edition.

• intransitive verb Biology To break down into component parts; rot.
• intransitive verb Physics To disintegrate in a process of radioactive decay or particle decay.
• intransitive verb Electronics To decrease gradually in magnitude. Used of voltage or current.
• intransitive verb Aerospace To decrease in orbit. Used of an artificial satellite.
• intransitive verb To fall into ruin.
• intransitive verb To decline in health or vigor; waste away.
• intransitive verb To decline from a state of normality, excellence, or prosperity; deteriorate.
• intransitive verb To cause to decay.
• noun The destruction or decomposition of organic matter as a result of bacterial or fungal action; rot.
• noun Rotted matter.
• noun Aerospace The decrease in orbital altitude of an artificial satellite as a result of conditions such as atmospheric drag.
• noun A gradual deterioration to an inferior state.
• noun A falling into ruin.

from The Century Dictionary.

• To pass gradually from a sound or perfect state to a less perfect state, or toward weakness or dissolution; fall into an inferior condition or state; specifically, become decomposed or corrupted; rot.
• Synonyms Putrefy, Corrupt, etc. See rot.
• To cause to become unsound or impaired; cause to deteriorate; impair; bring to a worse state.
• noun Gradual loss of soundness or perfection; a falling by degrees into an impaired condition or state; impairment in general; loss of strength, health, intellect, etc.
• noun Specifically Decomposition; putrefaction; rot.
• noun 3. Death; dissolution.
• noun . A disease; especially, consumption.
• noun . A cause of decay.
• noun Loss of fortune or property; misfortune; ruin : applied to persons.
• noun . plural Ruins.
• noun Synonyms Decline, decadence, deterioration, degeneracy, withering.

from the GNU version of the Collaborative International Dictionary of English.

• intransitive verb To pass gradually from a sound, prosperous, or perfect state, to one of imperfection, adversity, or dissolution; to waste away; to decline; to fail; to become weak, corrupt, or disintegrated; to rot; to perish
• transitive verb rare To cause to decay; to impair.
• transitive verb obsolete To destroy.
• noun Gradual failure of health, strength, soundness, prosperity, or of any species of excellence or perfection; tendency toward dissolution or extinction; corruption; rottenness; decline; deterioration
• noun obsolete Destruction; death.
• noun rare Cause of decay.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The process or result of being gradually decomposed.
• noun A deterioration of condition.
• verb intransitive To deteriorate, to get worse, to lose strength or health, to decline in quality.
• verb intransitive To rot, to go bad.
• verb intransitive, transitive, physics, chemistry To change by undergoing fission, by emitting radiation, or by capturing or losing one or more electrons.
• verb intransitive, transitive, physics To undergo optical decay, that is, to relax to a less excited state, usually by emitting a photon or phonon.
• verb transitive To cause to rot or deteriorate.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the process of gradually becoming inferior
• noun a gradual decrease; as of stored charge or current
• noun the spontaneous disintegration of a radioactive substance along with the emission of ionizing radiation
• verb undergo decay or decomposition
• verb lose a stored charge, magnetic flux, or current
• verb fall into decay or ruin
• noun the organic phenomenon of rotting
• noun an inferior state resulting from the process of decaying