Browser is usually short for web browser or perhaps HTML browser (but these days, lots of markup languages are supported like CSS, XHTML, some XML extensions, etc.). For most people this means mainstream browsers like Chrome, Firefox, and Safari, but can include so many others like Android's mobile browser (does it have a name?), Edge, Konqueror, Opera, etc. Some even use the dreaded Internet Explorer.
In terms of portability, the goal is to support the broadest range of browsers, assuming they support the standards correctly.