What is a Blog?

What is a Blog?
So what is a Blog anyway? I am asked every week via emails, conversation and Instant Messaging chats to define: 'what is a blog'. If you're reading this you may well be asking the same question.
There are a number of ways I could answer this question ranging from the broad to the highly technical.
Before I define the 'what is a blog' question -- here are a few definitions from other much wiser people to get us started:

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