(covers information from several alternate timelines)
A salt is any chemical compound composed of two ions that is not considered an acid or base. The word salt is also commonly used to refer to table salt, which is primarily sodium chloride.
Table salt, known chemically as sodium chloride (NaCl), is an essential chemical compound, vital for Earthly life. It is responsible for the salinity of bodily fluids, and is important for the balance of electrolytes. It is also the most common spice in the galaxy. (VOY: "Prototype") Salt was commonly dispensed in shakers.
Salt shakers in the shape of the USS Kelvin were, in 2255 of the alternate reality, featured at the Shipyard Bar in Iowa, on Earth. To use the salt stored inside, the salt was poured through holes in the container that formed an "s" shape on one side of the shaker, a forward area on the bottom of the model starship's saucer section. Shortly following a conversation between James T. Kirk and Christopher Pike during the bar's after-hours, Kirk absent-mindedly toyed with one of the Kelvin-type salt shakers, releasing salt from it. The salt shaker he fiddled with apparently had a missing component on its topside, where a screw jutted out of the miniature. (Star Trek)
In 2266, the USS Enterprise visited M-113 for routine medical examinations of the scientists working there. Ultimately, one of the scientists was revealed to be a species native to that arid world, a creature that required large quantities of salt to survive. This creature could draw the body salts from other beings to sustain itself, a process that was fatal to its victims. (TOS: "The Man Trap")
In 2377, when Seven of Nine served a meal that included luhvian quail as a main course, Tom Paris asked if there was any salt, but Seven insisted that "Additional seasoning is not required." (VOY: "The Void")