What is ketamine, the drug involved in Matthew Perry's death?

The investigation into the death of “Friends” star Matthew Perry has led to a sweeping indictment that pulled in five people who prosecutors say contributed to his ketamine overdose in October, including two doctors and a street dealer involved in providing Perry large amounts of the powerful anesthetic.

What is ketamine, the drug involved in Matthew Perry's death?
The investigation into the death of “Friends” star Matthew Perry has led to a sweeping indictment that pulled in five people who prosecutors say contributed to his ketamine overdose in October, including two doctors and a street dealer involved in providing Perry large amounts of the powerful anesthetic.