Here are some Web 2.0 definitions. In common seem to be the idea of a platform or platforms which mobilize collective co-creation capacity, and support on-demand availability of recombinable data and services.
Web 2.0 is the network as platform, spanning all connected devices; Web 2.0 applications are those that make the most of the intrinsic advantages of that platform: delivering software as a continually-updated service that gets better the more people use it, consuming and remixing data from multiple sources, including individual users, while providing their own data and services in a form that allows remixing by others, creating network effects through an "architecture of participation," and going beyond the page metaphor of Web 1.0 to deliver rich user experiences. [O'Reilly Radar > Web 2.0: Compact Definition?]John Hagel:
So, in the spirit of entering the conversation, let me suggest that Web 2.0 ultimately refers to "an emerging network-centric platform to support distributed, collaborative and cumulative creation by its users." [Edge Perspectives with John Hagel: What is Web 2.0?]He goes on to examine each of these attributes in turn. Richard MacManus includes the following in a longer discussion:
Web 2.0 at its most basic is using services on the Web. Some examples: Gmail for email, Flickr for photo-management, RSS for news delivery, eBay for shopping, Amazon for buying books. That's why the Web is being called a platform - because all of these services are being built and used on the Web. Why Web 2.0 only now though - hasn't Amazon been around since 1995? Why yes, but it's taken until 2005 for broadband and web technology to catch up and reach a 'tipping point' - the Web is fast becoming the platform of choice for developers, business, media, public services, and so on. [Read/Write Web]