Ask HN: What's the definition of a bot in social media?

There is much discussion of the twitter deal and about bots in social media, what percentage of user accounts are bots, etc. It occurred to me that if that case gets into the courts there will need to be a definition of what a bot is or isn't in order to ascertain percentages of user accounts.

What is your definition for a social media bot? Does it include the number of humans able to use a single account (e.g. customer service)? Does it include intent behind the account (e.g. product announcements)? With the anonymity of online personas, how can someone determine bot-or-not?