The term TOPIC OF INTEREST can be summarized as a RATE, which is a four letter word. A RATE is a measure of the relative speed or frequency at which something happens, such as the RATE of change in a variable over time or the RATE of interest on a loan. It can also refer to the cost of a service or product, such as the RATE of a hotel room or the RATE of a taxi ride. Rates are used in many different contexts and can be expressed in various ways, such as a percentage, a fraction, or a ratio.