a. A covering used to provide thermal insulation, as from a steam pipe.
b. A similar covering used to provide acoustical insulation, as on an air duct.
a. A plank or set of planks forming the outer surface of a centering for the construction of an arch.
b. Any of various structural members or materials used as a protective sheathing, as on the ceiling of a mine tunnel or around a boiler.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 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.