Merriam-Webster's Word of the Day for August 13, 2025, is calculus. Calculus is an advanced branch of mathematics concerning rates of change and calculating lengths, areas, and volumes. More broadly, it refers to the act of calculating or estimating using practical judgment. It can also mean solving or probing the meaning of something. For example, one might use their personal calculus to determine when an investment will pay off. Events can change the calculus of a situation, influencing decisions. The word entered English in the 17th century from Latin, initially meaning a pebble. These pebbles were used for counting and computation, linking the word to calculation. The Latin term likely derives from "calx," meaning lime or limestone, also the origin of the English word "chalk." Today, calculus encompasses mathematical principles, general problem-solving, and even a medical term for tartar on teeth.