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de·cay (dĭ-kā)
v. de·cayed, de·cay·ing, de·cays
1. Biology To break down into component parts; rot.
2. Physics To disintegrate in a process of radioactive decay or particle decay.
3. Electronics To decrease gradually in magnitude. Used of voltage or current.
4. Aerospace To decrease in orbit. Used of an artificial satellite.
5. To fall into ruin: a civilization that had begun to decay.
6. To decline in health or vigor; waste away.
7. To decline from a state of normality, excellence, or prosperity; deteriorate.
To cause to decay.
a. The destruction or decomposition of organic matter as a result of bacterial or fungal action; rot.
b. Rotted matter.
2. Physics
3. Aerospace The decrease in orbital altitude of an artificial satellite as a result of conditions such as atmospheric drag.
4. A gradual deterioration to an inferior state: tooth decay; urban decay.
5. A falling into ruin.

[Middle English decayen, from Old French decair, from Vulgar Latin *dēcadere : Latin dē-, de- + Latin cadere, to fall; see kad- in the Appendix of Indo-European roots.]

de·cayer n.

Synonyms: decay, rot, decompose, putrefy, spoil, molder, disintegrate
These verbs refer to the gradual process by which something breaks down or falls apart as a result of natural causes. Decay has wide application but often suggests partial deterioration short of complete destruction: "A decaying dam is an accident waiting to happen" (George Black).
Rot and decompose are closely synonymous with decay, but rot often emphasizes loss of structural integrity while decompose generally stresses breaking down into chemical components: The rotting timbers gave way under the added weight. When grass clippings decompose, they return nutrients to the soil.
Putrefy denotes an advanced stage of organic breakdown that is offensive to the senses: "Large numbers of cows and oxen ... were left to putrefy on mud flats after the floods receded, attracting rats" (John F. Burns).
Spoil usually refers to the process by which perishable substances become unfit for use or consumption: Fish will spoil quickly if not refrigerated.
To molder is to crumble to dust: The shawl had moldered away in the trunk.
Disintegrate refers to the reduction of something to particles, fragments, or constituent elements: The sandstone façade had disintegrated from exposure to wind and rain.

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2022 by HarperCollins Publishers. All rights reserved.

Indo-European & Semitic Roots Appendices

    Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:

    Indo-European Roots

    Semitic Roots

    The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.