The inclined surface of any part of the earth's surface.

1. The slope of a line through the points (x₁,y₁) and (x₂,y₂) is the number (y₂ - y₁)/(x₂ - x₁).

2. The slope of a curve at a point p is the slope of the tangent line to the curve at p.

The projection of a flight path in the vertical plane.