A specific person, not a demographic — specific enough that you could go and find ten of them.
Help me get specific about who my first customer is. Push me past demographics to an actual person: their role, what their week looks like, what makes this problem urgent for them, and what happens if they don't solve it. Keep pushing until it's specific enough to go find ten of them, then save who they are into my Market Analysis plan section with update_business_plan.