0.9% sodium chloride, commonly known as saline solution, is a mixture of salt and water. It contains 0.9 grams of sodium chloride per 100 milliliters of water, making it isotonic with human blood. This means it has a similar concentration of salts, which helps maintain fluid balance in the body. Saline solution is widely used in medical settings for various purposes, such as rehydrating patients, cleaning wounds, and delivering medications. It is also used in contact lens care and as a nasal rinse to relieve congestion. Its safety and effectiveness make it a staple in healthcare.