Saliva (commonly referred to as spit) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which DNA can be extracted), enzymes (such as lipase and amylase), antimicrobial agents (such as secretory IgA, and lysozymes).

The enzymes found in saliva are essential in beginning the process of digestion of dietary starches and fats. These enzymes also play a role in breaking down food particles entrapped within dental crevices, thus protecting teeth from bacterial decay. Saliva also performs a lubricating function, wetting food and permitting the initiation of swallowing, and protecting the oral mucosa from drying out.

Various animal species have special uses for saliva that go beyond predigestion. Some swifts use their gummy saliva to build nests. Aerodramus nests form the basis of bird's nest soup. Cobras, vipers, and certain other members of the venom clade hunt with venomous saliva injected by fangs. Some caterpillars produce silk fiber from silk proteins stored in modified salivary glands (which are unrelated to the vertebrate ones).