In the past, the definition of the term "blog" came from the abbreviation of "web log". This term describes a kind of online diary, because its entries are sorted by date.
Recently, I have seen that the term blog appears to be often used just to denote a single entry. For example, sometimes a single forum post is called "a blog" by some people who want just to denote this one particular entry. So in their view, a blog is something like a single reflection, a single "blurp" about something.
I think that this latter definition is wrong - it changes completely the meaning of the term "blogging" and reduces it to just another form of expressing one's opinion. Such a single post has a name: "blog post" or "blog entry". And in my opinion, this is how it should remain.