What defines the term "reaction rate"?

The term "reaction rate" is defined as the speed at which reactants are converted into products during a chemical reaction. This concept is crucial in understanding how quickly a reaction occurs, which can be influenced by various factors such as temperature, concentration, and the presence of catalysts. The reaction rate can be quantified and measured in terms of the change in concentration of reactants or products per unit time, providing a clear and measurable definition that aligns with option C.

In contrast, the other options relate to different aspects of chemical reactions but do not accurately define the reaction rate itself. For instance, the temperature at which a reaction occurs can influence its rate but does not define what the reaction rate is. The time taken to reach equilibrium pertains to the completion of a reaction rather than the speed of the individual reaction steps. Lastly, while the concentration of reactants over time might provide information that can be used to derive the reaction rate, it does not capture the essence of what reaction rate means on its own. Thus, option C correctly encompasses the concept of reaction rate.