LC₅₀ is short for "lethal concentration 50%" or "median lethal concentration". It is the concentration of a substance (in air or water) at which half the members of a population are killed after a specified duration of exposure. Exposure is typically through inhalation. This value is very important in toxicology and agriculture as it gives an indication of substance (eg. herbicide) toxicity. The LC₅₀ is inversely proportional to toxicity. A substance with a lower LC₅₀ is more toxic than one which has a higher LC₅₀.

Traditionally, the concentrations (x-value) are entered as mass of substance per volume of either air or solution. The units commonly used are milligram per liter (mg/L) or parts per million (ppm). The responses (y-value) are typically entered as percentage of population killed (range from 0 to 100). This will yield an upward-sloping sigmoidal curve. If percentage survival is used instead, a downward-sloping sigmoidal curve will be generated.

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