Salt peter typically refers to the chemical compound potassium nitrate, though it may also refer to sodium nitrate. Salt peter was once collected from decomposing material, but today, it is generally manufactured by treating sodium nitrate, mostly mined in Chile, with potassium chloride and collecting the precipitate. Salt peter was one of the ingredients of the first gunpowder, black powder. Today, it has many uses in both the laboratory and the larger world.

Black powder, oxidized with salt peter, is still used for small novelty explosives, such as fireworks and model rockets, though firearms typically use newer types of gunpowder. Salt peter is most widely used in manufacturing nitric acid, however, which is in turn used to make Trinitrotoluene (TNT) and other modern explosives. Nitric acid has many other applications as well, such as its use as a reagent in the laboratory and as an ingredient in plant fertilizers. Salt peter is also used as a fertilizer and as a stump remover, as it accelerates the decomposition process of tree stumps.

Salt peter can be found in many everyday products as well. It is used to preserve some foods, most commonly meats, although there are concerns about its health effects. Salt peter is also used in ice cream and toothpaste for sensitive teeth.

There used to be a widespread urban legend regarding salt peter that claimed it was added to the food in all male institutions, including the United States Army, as a way to curb libido. It is unlikely that this practice existed, however, since salt peter not only has no such effect, but also can have a number of ill side effects if taken in excess, such as poisoning, reproductive damage, and cancer.

Sodium nitrate, also called salt peter, shares many applications with potassium nitrate. Both can be used to manufacture nitric acid, to propel model rockets, and to increase the shelf life of meats and other foods. Sodium nitrate is also used to manufacture glass and enamel. Like potassium nitrate, sodium nitrate may increase one's likelihood of developing cancer, but it also occurs naturally in leafy green vegetables and may have some health benefits. A small study conducted in Sweden suggested that the nitrates found in vegetables may help lower blood pressure.