Caesium (Cs) is a metallic chemical element with atomic number 55. The only stable isotope of caesium is 133Cs, with 55 protons and 78 neutrons.
Since 1967, caesium is notably used as the official definition of how long a second is. To be precise, one second is the duration of 9 192 631 770 cycles of light being emitted by the caesium-133 atom. By extension, this is also used to officially define the length of a meter, which is defined as the distance that light travels in vacuum in a certain amount of time.
Caesium is often spelled cesium, but we're using the spelling caesium since it's the standard defined by IUPAC.