What is a Blog?

Interesting discussion going on in the BlogSphere as to what constitutes a Blog? Are there various categories of Blogs? Public, Private (Corporate & Personal)?.. If its a private Blog, then is it a Blog? Is there a officially accepted definition?

Robert Scoble sets out the following criteria which Jeff Sandquist disagrees.

1) Easy to do reverse-chronological content display. Type in a box and hit publish. New stuff goes at the top of the page. Old stuff moves down.
2) Discoverable. Through search engines
3) Social. I can track when you link to me from another domain, either through search engines, through trackbacks, or through my referrer logs.
4) Permalinkable. I can send you a link directly to a post.
5) Syndicatable. I can use a news aggregator to read your content, which lets me read a lot more Blogs.

My two cents:
Whether it is open or restricted, A Blog is something that exhibits the above characteristics.

Now, what do you call private blogs? (ie., public Blogs that are restricted)? That seems to be the point of contention between Jeff & Robert.

What's your take?