how well an item performs the activities expected of it.
How the system performs its functions.
A service provided to the user as part of a computer program that performs a specific task for the user. For example, you may want to discuss your need for "import functionality" with a programmer.
A generic term for the various tasks undertaken by a computer system. A single function would be "booking in" or "reporting".
Ability of a piece of software to perform the tasks that it was explicitly designed to perform, or implicitly needs to perform, efficiently and accessibly.