What is a Blog?

After doing a Google search for a query like, "What are blogs?" or "Definition of blog." For the benefit of those visitors, the following definitions are offered.

Dan Gilmour gives us this definition in the draft version of his book, Making the News :
So what is a weblog, anyway? Generally speaking, it's an online journal comprised of links and postings in reverse chronological order, meaning the most recent posting appears at the top of the page. As Meg Hourihan, co-founder of Pyra Labs, the blogging software company acquired by Google in February 2003, has noted, weblogs are “post-centric” -- the posting is the key unit -- rather than “page-centric,” as with more traditional websites. Weblogs typically link to other websites and blog postings, and many allow readers to comment on the original post, thereby allowing audience discussions.

Blogs run the gamut of topics. One may be a running commentary on current events in a specific arena. It may be a series of personal musings. It may be a purely political tract, as with Joshua Micah Marshall's TalkingPointsMemo.com. It may be pointers to other people's work or products, such as Gizmodo (www.gizmodo.com), a site devoted to the latest and greatest gadgets. Or it may be a constantly updated “what's new” by a domain expert, such as Glenn Fleishman's excellent WiFi news and commentary page. While some blogging software permits readers to post their own comments, this feature has to be turned on by the blogger, and a significant number of prominent bloggers have not enabled the comment feature. At the other extreme, the Slashdot weblog, featuring news about technology and tech policy, is essentially written by its audience.

What the best individual blogs tend to have in common is voice -- they are clearly written by human beings with genuine human passion.

That's a good, in-depth definition. Looking for something a little more concise?

Matisse Enzer's Glossary of Internet Terms puts it this way:
Blog -- (weB LOG) A blog is basically a journal that is available on the web. The activity of updating a blog is "blogging" and someone who keeps a blog is a "blogger." Blogs are typically updated daily using software that allows people with little or no technical background to update and maintain the blog.

Postings on a blog are almost always arranged in chronological order with the most recent additions featured most prominently.

The University of Lethbridge's Faculty of Education has a very brief definition.
Blog - A web log: an on-line diary or frequently updated personal web page.