1. The ability to perform a task or function; that set of functions that something is able or equipped to perform.
  2. In United States trademark law, the tendency of a product design to serve a function other than identification of the product, preventing that design from being protected as a trademark.
  3. The presence of a functional group.
    Aldrichimica Acta Volume 30 No 4 (pdf) from Sigma-Aldrich
    The vinyldiazomethanes that have been commonly used in our studies contain an electron-withdrawing group adjacent to the diazo functionality.

The above text is a snippet from Wiktionary: functionality
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