1. A flat plate, slab, or disk that is ornamented or engraved for mounting, as on a wall for decoration or on a monument for information.
2. A small pin or brooch worn as an ornament or a badge of membership.
3. A scaly patch on a body part, especially the skin.
4. A biofilm of bacteria in a matrix of polysaccharides and other substances on the surface of a tooth or teeth.
a. A deposit of fatty material on the inner lining of an arterial wall, characteristic of atherosclerosis.
b. The material that such a deposit is composed of.
6. A deposit consisting mainly of beta-amyloid and degenerating nerve tissue, found in the brain tissue of people with Alzheimer's disease.
7. A sharply defined zone of demyelination in the central nervous system characteristic of multiple sclerosis.
8. A clear, often round patch of lysed cells in an otherwise opaque layer of a bacteria or cell culture.
[French, from Middle French, metal plate, from plaquer, to apply (something such as gold or plaster) to a surface, from Middle Dutch placken, to coat, patch, paste; see PLACARD.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2019 by Houghton Mifflin Harcourt Publishing Company. 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:
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.