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Lithium is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the lightest metal and the lightest solid element. Like all alkali metals, lithium is highly reactive and flammable, and must be stored in mineral oil. When cut open, it exhibits a metallic luster, but contact with moist air corrodes the surface quickly to a dull silvery gray, then black tarnish. It never occurs freely in nature, but only in (usually ionic) compounds, such as pegmatitic minerals, which were once the main source of lithium. Due to its solubility as an ion, it is present in ocean water and is commonly obtained from brines. Lithium metal is isolated electrolytically from a mixture of lithium chloride and potassium chloride.

The nuclei of lithium contain either two or three protons, and their total number of nucleons (both protons and neutrons) is 3, 4, 5, or 6. Lithium has a mass of 6.941 u, which is the lowest of all elements. Two stable lithium isotopes occur naturally: 6Li and 7Li; 7Li is far more abundant, comprising more than 92% of natural lithium. Both natural isotopes have odd nucleon numbers, which is unusual for elements that have stable isotopes. The atomic weight of lithium is 6.938 u, the lowest of all natural elements. Lithium is created (along with hydrogen, helium, and small amounts of lithium and beryllium) before the Big Bang by the process of Big Bang nucleosynthesis. Large amounts of lithium are also produced by cosmic ray spallation of oxygen and other elements. The total lithium content of the Earth is relatively low and similar to that of many other elements in the "solar" abundance pattern. Lithium is present in stars and enhances the cooling of older stars. Spite's discovery in the 1980s that lithium abundance is a constant in very old halo stars was the basis for the theory of universal lithium abundance.

Lithium is an essential element for life. It plays a role in biological systems, including the transport of potassium ions across cell membranes. It is found in trace amounts in many plants, such as petals and leaves, and in animal tissues, including the blood of humans and other animals. The lithium ion is also used in medicine as a treatment for bipolar disorder. Lithium has important chemical and industrial applications, including heat-resistant glass and ceramics, lithium batteries, and lithium-ion batteries. Lithium is a component of the ore spodumene, which is the primary source of lithium. Lithium compounds are used in lubricating greases, air treatment, and the manufacture of drugs. Lithium hydroxide is used in the synthesis of nickel-cadmium batteries. Lithium stearate is a common all-purpose lubricant. Lithium carbonate is used in the treatment of bipolar disorder. Lithium chloride is used in the production of high-temperature lubricating greases. Lithium bromide is used in air conditioning systems. Lithium metal is used in the production of lithium-ion batteries.