What is the definition or description of: painkiller?

Painkiller: "painkiller" is a general term and not a medical term. Literally, it is something that "kills" the pain. That can include just about anything that is useful or effective.

Decreases pain: The basic definition of a "painkiller" is any medication that reduces pain. There are multiple "classes" of drugs that can act to decrease pain including non-steroidal anti-inflammatory drugs such as Ibuprofen and naprosyn, (naproxen) opiates such as codeine, morphine, and hydrocodone, anti-seizure medications such as gabapentin, and anti-depressants such as duloxetine. Your doctor should know which is best.