A biological assay corresponding to the minimum amount of a substance (expressed in g/kg) which is lethal to cats when injected IV. In practice, the LD50 for acute toxicity would be measured in rodents.
From what I can gather, cat units were last used by Western pharmacologists and physiologists in the mid-1940s. PETA–People for the Ethical Treatment of Animals has effectively squelched using cats in biological assays. A text (see reference) on Chinese traditional medicine refers to it as recently as 1998, which is hardly surprising, given that in China, cats are as likely to be à la carte as they are à la couch.
References Annual Review of Physiology March 1943; Vol. 5: 479-539
Amdur, J Doull, CD Klaassen, Eds, Toxicology, 4th ed, McGraw-Hill, New York, 1991
You-Ping Zhu, Chinese Materia Medica: Chemistry, Pharmacology and Applications CRC Press, 1998