Caesium (also known as Cesium) is a chemical element with the symbol Cs and atomic number 55. It is the heaviest alkali metal and is the last element in Group 1 of the periodic table. Caesium is a soft, silvery-gold metal that is highly reactive and flammable in air. It is the most reactive of all the alkali metals and has the lowest ionization energy.

Caesium is the 45th most abundant element in the Earth's crust, but it is quite rare and is only found in trace amounts. The main sources of caesium are the minerals pollucite and lepidolite. Caesium can also be obtained by electrolysis of molten caesium chloride.

Caesium has a wide variety of uses, including:

* In atomic clocks and frequency standards, where its extremely precise frequency makes it ideal for timekeeping.

* In photoelectric cells, where its low work function makes it sensitive to light.

* In medical imaging, where it is used as a contrast agent in X-rays and CT scans.

* In ion propulsion systems for spacecraft, where its low ionization energy makes it an efficient propellant.

Caesium is a hazardous material and can be toxic if ingested or inhaled. It is also highly flammable and can easily catch fire in air. Caesium should only be handled by trained professionals.