A healing chem. When injected, the chem provides immediate healing of minor wounds.
”— In-game description
Stimpak, or stimulation delivery package, is a type of hand-held medication used for healing the body. Superior versions are super stimpak and ultra stimpak.
This item consists of a syringe for containing and delivering the medication and a gauge for measuring the status of the stimpak's contents. When the medicine is injected, it provides immediate healing of the body's minor wounds.
In Fallout and Fallout 2, stimpaks only restore Hit Points. Crippled limbs have to be restored with use of the Doctor skill. They are plentiful and easy to come by in more civilized areas, although on the outskirts of post nuclear civilization (such as Klamath or Den in Fallout 2), they can be quite rare and fetch high prices.