Density is a device about the size of a deck of cards. When you plug it in to an outlet, it will immediately count the number of people in a room. The longer it listens, the better its data becomes: measuring visitor count, new v. loyal customers, visitor duration, etc.
It does this by listening for a smartphone's WiFi address or MAC address. It's anonymous, so it doesn't know that it's you or me, just that a unique individual is there.
Density is a device, a native application (iOS for now), and analytics. The system is sold to local, independently owned businesses in cities. The sensor is shipped to an owner, installation is handled by the native application pictured above, which doubles as the delivery mechanism for Density's movement analytics.
What's remarkable about Density is its network. When installed in a sufficient number of locations across a city, Density can identify referral traffic, businesses with similar movement profiles, and other points of similarity.