To use a lower or the next-lowest number, especially so as to eliminate decimal places. (Usually done when the non-whole number is less than .5; higher than that, and the number would typically be rounded up.) A noun or pronoun can be used between "round" and "down." Most stores round their prices down to .99 instead of keeping them as whole numbers. The accountant had been using the exact numbers from the sales data, while I had been rounding them down when doing my own calculations.