1. The act of extending or the condition of being extended: the extension of the subway into the suburbs.
2. The amount, degree, or range to which something extends or can extend: The wire has an extension of 50 feet.
a. The act of straightening or extending a limb.
b. The position assumed by an extended limb.
4. A length of human or synthetic hair attached to the scalp or to strands of hair close to the scalp to add volume, length, or color.
5. Medicine The application of traction to a fractured or dislocated limb to restore the normal position.
a. An addition that increases the area, influence, operation, or contents of something: an extension for the vacuum cleaner; built a new extension onto the hospital wing.
b. An additional telephone connected to a main line.
a. An allowance of extra time, as for the repayment of a debt.
b. The period of this extra time: three months' extension on the loan.
8. The property of an object by which it occupies space.
a. A program in a university, college, or school offering academic instruction to nontraditional students, such as working adults, who cannot attend classes at the usual place and time.
b. A publicly funded program offering such instruction along with information on agriculture, home economics, and business.
10. Computers A set of characters that follow a filename and are separated from it by a period, used to identify the kind of file: In most operating systems, filenames having the extension .EXE are executable files.
11. Logic The class of objects designated by a specific term or concept; denotation.
12. Mathematics A set that includes a given and similar set as a subset.
[Middle English extensioun, from Old French extension, from Latin extēnsiō, extēnsiōn-, from extēnsus, past participle of extendere, to extend; see EXTEND.]
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:
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.