noun

• (uncountable) The ability to do a task, performance, or execution; a set of functions that something is able or equipped to perform.
  "Glu Mobile, who realigned its workforce towards centralized GaaS functions last year, announced earlier this month that it will shut down GameSpy Technology on May 31. The service provides multiplayer functionality for dozens of games on consoles, computers and mobile devices."
• (US, law, uncountable) In trademark law, the tendency of a product design to serve a function other than the identification of the product, preventing that design from being protected as a trademark.
• (organic chemistry, countable) The presence of a functional group.
  "To appreciate the tasks and problems arising in the synthesis of the monomers it is of fundamental importance to understand that the process used must be capable of yielding organosilanes with different functionalities, since all functionalities are required for synthesis of silicones."