A paved, typically multi-lane, road designed for high-speed or long distance motor vehicle traffic, often between cities. They're main public roads serving as high-speed thoroughfares.
The difference between a highway and a street is that a street is usually inside a city and designed for both vehicle and pedestrian traffic, with traffic lights, sidewalks, and crosswalks, while a highway is designed only for vehicles and doesn't have these features. A street is usually designed to allow access to nearby buildings, while a highway usually has limited entry and exit points.