In this testing procedure, laboratory animals, usually rats, are given measured doses of compounds until approximately half of the test population die.
Chronic (long term) toxic doses and dermal (high-dose topical applications) have also been studied with laboratory animals.
Toxic chronic doses are always less than the corresponding acute dose.
We should consider (outside of ethical considerations - but no effective substitute has yet to be found) that such tests generally are based on either acute oral (by mouth) or injected lethal doses.
This means that the LD50 dose represents the median toxic dose taken all at one time, either by ingesting or by direct injection of the test compound.
The vast majority of Aromatherapy books are written for the lay public.