Jargon: Web 2.0 (noun or adjective).

Definitions:
1. Generally refers to a second generation of services available on the World Wide Web that let people collaborate, and share information online (Source: Wikipedia).
2. Web 2.0 is the network as platform, spanning all connected devices (Source: Tim O'Reilly of O'Reilly Publishing, who runs the Web 2.0 conference).
3. With its allusion to the version numbers that commonly designate software upgrades, Web 2.0 was a trendy way to indicate an improved form of the World Wide Web (also from Wikipedia).
4. Web 2.0 is the latest moniker in an endless effort to reignite the dot-com mania of the late 1990s (Source: John Dvorak of PC Magazine).
5. It's a technology upgrade, one that finally does what they'd said version 1.0 would do (Source: Paul Boutin of Slate).

For the long-form definition, check out O'Reilly's essay, What is Web 2.0.

Origin: According to O'Reilly in his essay: "The concept of 'Web 2.0' began with a conference brainstorming session between O'Reilly and MediaLive International. Dale Dougherty, web pioneer and O'Reilly VP, noted that far from having 'crashed,' the web was more important than ever, with exciting new applications and sites popping up with surprising regularity. What's more, the companies that had survived the collapse seemed to have some things in common. Could it be that the dot-com collapse marked some kind of turning point for the web, such that a call to action such as 'Web 2.0' might make sense? We agreed that it did, and so the Web 2.0 Conference was born."

Web 2.0 Companies:

Bloglines
del.icio.us
Flickr
MySpace
Technorati
YouTube
Wikipedia

Web 1.0 Companies:

Amazon
eBay
Friendster
Lycos
Netscape
Pets.com
Webvan

Synonyms: The Living Web, Read/Write Web, The Live Web, Semantic Web, The New New Thing, Bubble 2.0, Media 2.0, Distributed Web

Antonyms:: Web 1.0, Dot-Com Boom, The Bubble, Closed Systems, Data Silos

Common usage:
1. We are living in the era of Web 2.0, where people collaborate more online, and create and annotate more media than ever before.

2. That Internet startup is pitching itself to venture capitalists as a Web 2.0 company that is using the wisdom of crowds and user-generated content in order to get the attention of bigger players such as Yahoo and Google, who might eventually buy it out.

3. If one more company calls itself a part of Web 2.0, I'm going to puke!

Love it:
Tim O'Reilly of O'Reilly Publishing
John Battelle of FM Publishing (also runs Web 2.0 conference)
Michael Arrington of TechCrunch
Ross Mayfield of SocialText
Susan Mernit, new media consultant

Hate it:
John Dvorak of PC Magazine
Paul Boutin of Slate
Andrew Orlowski of The Register
Russell Shaw of ZDNet
Dave Winer of Scripting News

The last word:
Web 2.0 might just be a nerdy term for the new, revamped, post-bubble Internet, where profits mean a little more than just getting eyeballs, and where blogs and social media are transforming our lives. But should you really care about the term? Not necessarily.

"Ask your mom and dad if they use any of the popular Web 2.0 services," writes Hooman Radfar on the O'Reilly Radar blog. "Ask folks at Fortune 500 companies what their strategy is to react to the evolution of Web 2.0. Ask your friends if they have invested in any Web 2.0 company's stock, or are excited about the exciting new direction that the web has taken. They will look at you like you are crazy. And, the first question they will undoubtedly ask is -- what is Web 2.0?"