I'm looking for a clear, single-sentence definition of usability.
A short definition of usability would make it much easier to convince people that usability is worth investing in. For example, a recent proposal for a Usability Roadshow to promote usability throughout the organisation was met with the obvious question: "OK, but what is usability exactly?"
Words like "effective", "efficient" and even "enjoyable" are often mentioned. But I still haven't seen a definition that I feel captures the essence of usability. I would like your help to come up with a short, standard definition to use next time somebody asks "What is usability?"