The wire has an extension of 50 feet.
An extension to a factory.
Extension ladder, extension cord.
The extension of the subway into the suburbs.
An example of an extension is adding a second story to a single story house.
An example of an extension is line two on a business line.
An example of an extension is someone getting an extra fifteen days to pay their electric bill.
In most operating systems, filenames having the extension .EXE are executable files.
An extension for the vacuum cleaner; built a new extension onto the hospital wing.
Three months' extension on the loan.
Files with the .txt extension usually contain text.