What is this organ found in bees, wasps, bumblebees and hornets called?

A stinger (or sting) is a sharp organ found in various animals (typically insects and other arthropods) capable of injecting venom, usually by piercing the epidermis of another animal.

Stinging insects produce a painful swelling of the skin, the severity of the lesion varying according to the location of the sting, the identity of the insect and the sensitivity of the subject. Many species of bees and wasps have two poison glands, one gland secreting a toxin in which formic acid is one recognized constituent, and the other secreting an alkaline neurotoxin; acting independently, each toxin is rather mild, but when they combine through the sting, the combination has strong irritating properties. In a small number of cases, the second occasion of a bee or wasp sting causes a severe allergic reaction known as anaphylaxis.

Hornets, some ants, centipedes, and scorpions also sting.