查阅《麦克米伦高阶美语词典》（英语版）可知，“round down”表示“to reduce a number to the nearest whole number, or the nearest number ending in zero”，给出的例句为“The total came to $10.04, but she rounded it down to $10.”，通过这个例句可看出，“round down”就是数字向下舍入向整数靠拢。以0.4567为例，round down to 3 decimal places，即为0.456。

“Round up”正好相反，表示“to increase a number to the nearest whole number, or the nearest number ending in zero”，就是数字向上舍入向整数靠拢。以0.1234为例，round up to 3 decimal places，即为0.124。

真正的四舍五入是“round off”，词典给出的释义为“to change a number to the nearest whole number, or the nearest number ending in zero”，例句为“Well that comes to 169, so let’s round it off to 170.”。还以0.4567为例，round off to 3 decimal places，即为0.457。