What does file name extension mean?

Here's a list of possible definitions for the term file name extension:

extension, filename extension, file name extension(noun)
a string of characters beginning with a period and followed by one or more letters; the optional second part of a PC computer filename
"most applications provide extensions for the files they create"; "most BASIC files use the filename extension .BAS"

file name extension
A filename extension, file name extension or file extension is a suffix to the name of a computer file (e.g., .txt, .docx, .md). The extension indicates a characteristic of the file contents or its intended use. A filename extension is typically delimited from the rest of the filename with a full stop (period), but in some systems it is separated with spaces. Other extension formats include dashes and/or underscores on early versions of Linux and some versions of IBM AIX.Some file systems implement filename extensions as a feature of the file system itself and may limit the length and format of the extension, while others treat filename extensions as part of the filename without special distinction.

Chaldean Numerology
The numerical value of file name extension in Chaldean Numerology is: 9
Pythagorean Numerology
The numerical value of file name extension in Pythagorean Numerology is: 1