Is this defined in terms of population? A writing system? Traditions? What's the difference between a bunch of people in the same geographic area and a civilization?
Hi,
Transhistorical questions like this are a much better fit for the experts at /r/AskAnthropology or perhaps even /r/AskSocialScience.
In the present day?
Well, whatever you like.
You can fine some good discussion on the term civilization from an earlier thread here. In current discussion, it's a vague term used to refer to a culturally uniform group of people. We'll say "Mesoamerican civ" or "Andean civ" or "Greco-Roman civ" because the people and cultures within them have more in common with each other than they do with those outside them. It's not a concept that most scholars read much into. It looks nicer than "cultural complex" and is broader than "society." More than anything, it's a term we avoid due to its troubled history tied up with Eurocentric social evoultionary ideas that did try to classify societies into hierarchies (see Lewis Henry Morgan's Ancient Society)