It is measured by the amount it takes to kill half a population of an animal or an organism. The animal used most often is mice.
Science Learning HubToxicity can be measured by the effect the substance has on an organism, a tissue or a cell. We know that individuals will respond differently to the same dose of a substance because of a number of factors including their gender, age and body weight. Therefore a population-level measure of toxicity is often used. The probability of an outcome for a population is then related to a given individual in a population.
https://www.sciencelearn.org.nz/resources/366-measuring-toxicity