Here are some ways to define the format/genre/communication channel... etc.
I usually go back to Mortensen, Torill and Walker, Jill. (2002). Blogging thoughts: Personal publication as an online research tool. In A. Morrison ed. (Ed.), Researching ICTs in context (1 ed.) (pp. 249). Oslo, Norway: InterMedia University of Oslo, Norway. pdf online. That's where I got the "reverse chronologically arranged collection of discrete posts" idea that I use (actually, I can't check to make sure that quote is exact because M's computer can't open pdfs!)
A "web log", or "blog" for short, lets you write about whatever
you want and make it visible online. Your entries are displayed in reverse-chronological order, people reading them can post comments, and you can reply to those comments. I could describe how to set up a blog, but that would make it seem harder than it is. Any idiot can do it, and many do. Websites like Wordpress and Blogger will lead you through the process step by step-- and they're free.
Bonetta in Cell just says:
Blogs provide an online discussion forum for issues of current interest and are updated regularly with new short articles on which readers can comment.
Boulos, Maramba, and Wheeler in BMC Medical Education:
A related Web information sharing technology is the 'blog'. A blog (WeBLOG) is a Web site that contains dated entries in reverse chronological order (most recent first) about a particular topic . Functioning as an online journal, blogs can be written by one person or a group of contributors. Entries contain commentary and links to other Web sites, and images as well as a search facility may also be included.
Because blogs engage people in knowledge sharing, reflection, and debate, they often attract a large and dedicated readership . They can also engender the drawing together of small virtual groupings of individuals interested in co-constructing knowledge around a common topic within a community of practice.
Standard blog features include easy posting, archives of previous posts, and a standalone Web page for each post to the blog with a unique URL. The latter feature facilitates linking to and organising content within the same blog and from external sites .
Blog is software. The rest is commentary.
You may also use "A weblog (âblogâ) is an publication on the WorldWideWeb in which brief entries are displayed in date order, much like a diary or journal." from Adam M. Goldstein as it is Open Access.
Or "Blogs are frequently updated web pages on which authors post short comments about news items, interesting websites, their thoughts and more." from Anton
I was going to add to this by saying, well, what Coturnix did, who beat me to it. I wish I had known about the Mortenson and Torill paper, but I hadn't come across it, in which the connection with reverse chronological order is pointed out.
"reverse chronologically arranged collection of discrete posts" is exactly what I use when writing about blogs offline, although I'd never seen it anywhere else before. But it rather precisely describes a blog's essential points.
Here's how I defined it in "Blogs and libraries" on the Library Leadership Network:
A blog is a web-based set of individual posts, by one individual or a defined group, initially presented to readers in reverse chronological order--that is, newest first.
After which I list some of the other elements typically included in definitions, and why I omit them as being wrong or overly narrow.