In medicine, saline (also saline solution) is a general phrase referring to a sterile solution of sodium chloride (NaCl, more commonly known as table salt) in water, but is only sterile when it is to be placed parenterally (such as intravenously); otherwise, a saline solution is a salt water solution. The sterile solution is typically used for intravenous infusion, rinsing contact lenses, nasal irrigation, and cleaning a new piercing. Saline solutions are available in various formulations for different purposes. Salines are also used in cell biology, molecular biology, and biochemistry experiments.

Normal saline (NS or N/S) is the commonly used phrase for a solution of 0.90% w/v of NaCl, about 300 mOsm/L or 9.0 g per liter. Less commonly, this solution is referred to as physiological saline or isotonic saline, neither of which is technically accurate. Normal saline was invented by Dutch physiologist Hartog Hamburger in 1896. It is used frequently in intravenous drips (IVs) for patients who cannot take fluids orally and have developed or are in danger of developing dehydration or hypovolemia. NS is also used for aseptic purpose. NS is typically the first fluid used when hypovolemia is severe enough to threaten the adequacy of blood circulation, and has long been believed to be the safest fluid to give quickly in large volumes. However, it is now known that rapid infusion of NS can cause metabolic acidosis.