Web usability is the application of usability in those domains where web browsing can be considered as a general paradigm (or "metaphor") for constructing a GUI.
Web usability is a general approach: it is as much about the effectiveness of transferring information via the Internet, as it is about the smooth interaction of an end-user with online (and offline) software.
With web browsers becoming a greater part of the interaction between humans and electronic devices, the terms "usability" and "web usability" overlap in numerous ways.
As more research results about usability become available, this leads to the development of methodologies for enhancing web-usability.
In some contexts the meaning of web-usability is narrowed down to efficiency for e-commerce websites (i.e. the efficiency in triggering sales and/or performing other business-like transactions). This probably historically derives from the renewed attention web usability received at the time many early 21st century web commerces started to fail.
Where during the emergence of internet in the last decades of the 20th century fancy graphical design had been regarded as indispensable for a successful e-business application, web-usability protagonists said quite the reverse was true: principles like the KISS principle (from the end-user's viewpoint) had proven to be much more indispensable for such success.