Boil: A skin abscess that forms at a hair follicle infected with pus-forming bacteria. The main treatments include hot packs and draining (lancing) the boil when it is soft. Antibiotics are usually not very helpful in treating boils. A person who has a fever or long-term illness, such as cancer or diabetes, or is taking medications that suppress the immune system should contact a health care practitioner on developing a boil. Also known as furuncle.