file extension

/F AY1 L IH0 K S T EH0 N SH AH0 N/
noun
  1. 1

    A short series of text characters at the end of a computer filename, used to indicate the type of contents and the software that will be required to operate or open it.

    A text file may have a file extension of .txt.

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Source: https://en.wiktionary.org/wiki/file%20extension