The general definition is the ratio of the longer to the shorter dimension of a rectangle, i.e., L/B
The term has specific usages. In aerodynamics it is the ratio of the span of a wing to its width or the ratio of the square of the span to total area. In relation to television screens it is the ratio of the frame width to height. In the motor industry it is used for the ratio of height of the tyre to its width, otherwise called the tyre profile.