In large part, what makes something usable is the absence of frustration in using it.
True usability is invisible. If something is going well, you don't notice it. If the temperature in the room is comfortable, no one complains. But usability in product happens along continuum. How usable is your product? Could it be more usable even though users can accomplish their goals? Is it worth improving?
Though there are quantitative approaches to testing the usability of products, it's impossible to measure the usability of something. You can only measure how unusable it is: how many problems people have using something, what problems are and why.
(c) Handbook of Usability Testing. How to Plan, Design and Condust Effective Tests. Second Edition. J.Rubin, D.Chisnell.