WordNet (r) 3.0 (2006):

usability
    n 1: the quality of being able to provide good service [syn:
         serviceability, serviceableness, usableness,
         useableness, usability]

The Free On-line Dictionary of Computing (30 December 2018):

usability

    The effectiveness, efficiency, and satisfaction
   with which users can achieve tasks in a particular environment
   of a product.  High usability means a system is: easy to learn
   and remember; efficient, visually pleasing and fun to use; and
   quick to recover from errors.

   (http://orrnet.com/).

   (1999-04-01)