Chapter 2 of Confessions of a Recovering Engineer: The Strong Towns Approach to Transportation defines a street as:
Street: A platform for building community wealth.
This is in contrast to a road, which is a high-speed connection between two places. It is further elaborated in Confessions of a Recovering Engineer:
While roads connect places, streets are the framework for building a place. Streets provide the greatest value when they create places people want to be. When people choose to buy land, build something on it, then maintain and improve what is built over time, they are building measurable wealth within the community. The most accurate measurement for the value of a street is the financial productivity of the land adjacent to it. How much value is created per acre of land that abuts the street?
Note that the purpose of a street is not transportation or mobility. A street can accommodate transportation and provide mobility, but that is not the purpose any more than a shopping aisle's purpose is to get a shopper from one side of the store to the other.