() a measurement of how much of a substance a liquid or other substance contains 百万分之…， 百万分率（浓度检测单位）◆air with 50 ppm nitrogen dioxide 含 50ppm 二氧化氮的气体
() a measurement of how efficient a manufacturing process is, which records the number of defects (= faults in the way sth has been made) found in each million parts produced 每百万件产品的次品率◆We aim for a quality level of 3ppm in our production line. 我们的目标是把生产线上的次品率控制在百万分之三。