probably the most accurate definition is "something that might have previously been implemented as a desktop app or in flash, but is mostly in html/css/js" htm

probably the most accurate definition is "something that might have previously been implemented as a desktop app or in flash, but is mostly in html/css/js" html is now versionless, html5 is a nice marketing term that we can just use to talk about "new cool shiny toys", dont worry about a strict definition