The speed at which a variable changes over a specific period of time. Rate of change is often used when speaking about momentum, and it can generally be expressed as a ratio between a change in one variable relative to a corresponding change in another. Graphically, the rate of change is represented by the slope of a line.
Rate of change is often illustrated by the Greek letter delta. Many traders pay close attention to the speed at which one variable changes relative to another. For example, option traders study the relationship between the rate of change in the price of an option relative to a small change in the price of the underlying asset, known as an options delta.