Calculus is often referred to as "The Calculus" because it represents a specific branch of mathematics distinguished by its methods for calculating rates of change and areas under curves. The term "calculus" originates from the Latin word for "pebble," which was used in ancient Roman counting devices. The distinction of "The Calculus" emphasizes its significance compared to other mathematical disciplines, which do not typically use the definite article. Additionally, the discussion touches on the historical context of the term and its evolution, noting that it was once called "the infinitesimal calculus." Overall, the use of "The" adds a sense of importance and specificity to this mathematical field.