If someone asks, “How are you?” and you reply, “Fine,” without really thinking about it, that’s an automatic reply. Something that’s automatic is done without thinking, almost as if you’re a well-programmed machine.
The word automatic can be used to describe a machine that works on its own, performing its tasks with little or no human intervention, such as an automatic transmission on a car. Or, automatic can describe a person whose actions are like a machine's. If you flash a smile every time someone walks through the door, it’s an automatic smile, a natural reaction you didn't think about or plan in advance.