What does extension mean?
Extension is a noun describing both an action and a thing: it can mean the act of making something longer, larger, or longer-lasting — as in an extension of a deadline or contract — or the item itself that brings about such growth, such as a new wing built onto a house. The word carries no strong connotation of its own; its tone comes from context. A deadline extension is often welcome relief, while an extension of a war suggests unwelcome prolongation. Modern life has given the word several specialized senses: a phone extension is a secondary line connected to a main system, and a file extension is the short suffix after a dot that tells computers what kind of file they are dealing with. In philosophy and logic, an extension is the set of real-world things a concept actually covers. Derived from the Latin extendere, "to stretch out," the word neatly embodies its own meaning: it stretches a familiar idea across many domains.
nounThe act or process of extending, or the state of being extended; something added to enlarge or prolong an original thing.
- An act of prolonging or enlarging something in time or space (e.g., an extension of a deadline).
- A part added to an existing building or structure to increase its size.
- A telephone line connected to a main switchboard; also the device itself.
- A suffix appended to a filename indicating its format (e.g., .txt).
- In logic and semantics: the set of things a term applies to, as opposed to its intension.
"The deadline was extended by a two-week extension granted to all applicants."
"She requested an extension on her essay because of illness."
"They are planning a kitchen extension that will open onto the garden."
Regular plural; commonly used when referring to multiple phone lines, file types, or additions.
"All three extensions on the ground floor were busy."
That ".jpg" at the end of your files and the extra room built onto your house are both "extensions" — one Latin root, stretched across five centuries of English.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of extension
Extension derives from the Latin extensio, meaning "a stretching out," formed from extendere (ex-, "out," plus tendere, "to stretch"). It entered English via Old French around the late Middle Ages, with senses relating to lengthening and enlargement developing over the following centuries. It shares this root with related words such as extent, extensive, extend, and pretense, all ultimately tracing back to the same Latin verb tendere, "to stretch."
Related word forms
How extension is actually used
Neutral in register, used freely in both formal and everyday contexts. In British English especially common for home additions (a "house extension"). The computing sense (file extension) is standard technical usage worldwide.
Easily confused with extension
"Extent" refers to the degree or scope of something (the extent of the damage), while "extension" refers to the act of stretching or adding on (an extension of the deadline).