Potassium is one of the chemical elements — a soft, silver-white metal. Its symbol is K. You will almost never see it as a metal, because pure potassium reacts violently the moment it touches water or even damp air; it catches fire and jumps around. Laboratories keep it under oil to stop that. So when people say 'potassium' in everyday life, they mean the potassium inside some compound, not a lump of shining metal.

The word shows up in two very different rooms. In farming and industry, potassium is the third of the big three plant foods, sold as potash. In health and food, potassium is a mineral your body must have — it helps your heart beat steadily, helps muscles work, and works together with sodium to control the water in your body. Doctors talk about potassium levels in the blood being too high or too low, and both are dangerous. Bananas are the famous example, but beans, potatoes and dark leafy vegetables actually carry more.

One thing that puzzles students: why is the symbol K and not P? Because Latin-using scientists called the substance kalium, from the Arabic word for the plant ash it came from, and Germany and much of Europe kept that name. English took Humphry Davy's 'potassium' instead, but the international symbol stayed K. So the letter and the English word come from two different naming traditions. (P was already taken by phosphorus anyway.)

It is a mass noun: 'potassium', never 'a potassium' or 'potassiums'. Before another noun it is common — potassium chloride, potassium levels, a potassium supplement.