Salt, also known as sodium chloride, (TV: Four to Doomsday) was a mineral that was used on Earth as a preservative and flavour enhancer.
The First Doctor asked the Sensorites for salt when Ian Chesterton collapsed from Atropa belladonna poisoning. He was able to use salt and water as a remedy for Ian. (TV: "A Race Against Death")
Due to its unique chemical properties, disrupting the osmotic balance and electrical conductivity of their cells, table salt could act as a defence against the Fendahl. The Fourth Doctor could kill Fendahleen by firing table salt out of a shotgun. The power of salt to ward off evil, then, entered into human folklore in the form of throwing salt over one's shoulder for good luck. (TV: Image of the Fendahl) The Hervoken were also similarly vulnerable to salt. (PROSE: Forever Autumn)
Around 6100 BC Osirans utilised salt-based spacecraft. At this time, Sekhmet was entombed in a spaceship made from trisilicate, which was "the hardest form of salt in the galaxy". In Earth "old wives' tales", salt was used to ward off "evil". (AUDIO: The Bride of Peladon)
After spilling salt at dinner, Winifred Gillyflower fed some to Mister Sweet. (TV: The Crimson Horror)
After rejecting pure salt for being "too salty", the Tenth Doctor ate anchovies containing salt as part of a treatment to eject cyanide from his body. (TV: The Unicorn and the Wasp)
Adric annoyed Tegan by requesting that she pass him "the sodium chloride". At first, she did not know what he meant. (TV: Four to Doomsday) The Fourth Doctor also expressed annoyance at this, believing Adric to be showing off his intelligence. (AUDIO: A Full Life)
Eating a portion of fries that had passed through the Arc of Infinity, Omega was disappointed to find that they had no salt on them. (PROSE: Anti-Matter with Fries)
Clyde Langer suggested that the Slitheen were allergic to the salt in his cold chip sandwich. Luke Smith noted that if the Slitheen were made mostly of water it would dehydrate them like slugs. However, that theory was debunked as salt was contained in the bread and water of the sandwich another Slitheen ate. It was soon realised that the Slitheen were calcium-based and allergic to vinegar. (TV: Revenge of the Slitheen)
Salt 'n' vinegar was a flavour of crisps. (PROSE: Citation Needed)
Salt was the currency of the Sontila. (AUDIO: The Doctor's Coat)