ISO 9241 standard definition of usability:
"The effectiveness, efficiency, and satisfaction with which specified users achieve specified goals in particular environments."
"Usability rules the web. Simply stated, if the customer can't find a product, then he or she will not buy it. The web is the ultimate customer-empowering environment. He or she who clicks the mouse gets to decide everything. It is so easy to go elsewhere; all the competitors in the world are but a mouse-click away."
Reference:
Nielsen, Designing web usability, 2000.