graceful degradation and progressive enhancement
what is graceful degradation and progressive enhancement?
Though there are nuances of meaning, when applied to web design they both mean essencially the same thing: making sure that when you use techniques and software not universally supported that the core content of the site is still accessible.
This usually applies to making sure that when you use advanced javascript and flash behaviors and interactive experiences that the core or the content is still accessable when users have these things disabled. For JavaScript this usually means proper coding technique that use 'hooks' into your code instead of using inline scripts or other less refined methods. That tend to clutter your code and break when a browser that doesn't support a feature views your page.