A turn is a particular measure of how much something has rotated or turned around a point. It is often measured in degrees or "angle" to describe how much the object has changed it's direction. When you turn around in a circle, you make 360 degrees of turn. That's like turning around all the way to where you started. A smaller turn would be less than 360 degrees, like turning 90 degrees, which is a quarter of the way around.