Other forms: extensions
An extension is a part added on to make something bigger or longer. The room built onto the back of a house, the extra fortnight you get for an essay, the cable that lets a lamp reach the far wall.
The verb is extend, and extension is the name for whatever extending gives you. On an office phone system your extension is the short internal number people dial. On a computer the extension is the part of a filename after the dot — .pdf, .csv — and a browser extension is a small add-on that bolts extra features onto Chrome or Firefox. Hair extensions are lengths of false hair clipped or glued in. The building sense is very British; Americans say addition. And there is an abstract use: an extension of an argument is that argument stretched to cover a new case.
1nounextensionA new part built onto an existing building.
“They built a glass extension across the back of the house.”
synonyms: annexe, addition
2nountimeExtra time allowed beyond a deadline.
“She asked for a week's extension on the essay.”
3nounshortA short internal telephone number inside an organisation.
“Call me on extension 214 and I'll come down.”
4nounfilenameThe part of a filename after the dot, showing what kind of file it is.
“The file would not open because the extension was wrong.”
5nounsmallA small add-on that gives a program or browser extra features.
“He installed an extension that blocks adverts.”
6nounactThe act of making something longer, larger or wider in reach.
“The extension of the railway took four years.”
extension is an added part.
Latin had extensiō, Old French made it estension, and Middle English wrote it extensioun — close enough to today's word to read at a glance. The verb sitting behind it is extend, and every use of the noun is that same stretch: the room on the back, the extra week, the number on the phone.
Latin
extensiō
Old French
estension
Middle English
extensioun
English, today
extension
From Middle English extensioun, from Old French estension, from Latin extensiō.
Etymology from Wiktionary, CC BY-SA