To express a number as a lower or the next-lowest number, especially so as to eliminate decimal places in a non-whole number. (Usually done when the non-whole portion is less than 0.5; decimals of 0.5 and up would typically be rounded up.) A noun or pronoun can be used between "round" and "down." The bill came out to $41.73, but they were nice enough to just round it down to $40 even. You can't just round these figures down to whole numbers—you've got to report them exactly as they are.