1. n. 
The lethal concentration of a substance, reported in ppm, that kills 50% of a population of test organisms, such as mysid shrimp, in a standard, controlled laboratory bioassay test. In offshore drilling operations, the LC50 number is used to determine whether waste mud or cuttings can be discharged into the water. The larger the LC50 ppm number from the test, the less toxic the sample is to the organism. For example, if LC50 number is 1,000,000 ppm, the sample is presumably nontoxic according to the test protocol.