Lutetium is a chemical element with the symbol Lu and atomic number 71. It is in the d-block of the periodic table, not the f-block, but the IUPAC classifies it as a lanthanide.
It is one of the elements that traditionally were included in the classification, "rare earths". One of its radioactive isotopes (176Lu) is used in nuclear technology to determine the age of meteorites. Lutetium usually occurs in association with the element yttrium and is sometimes used in metal alloys and as a catalyst in various chemical reactions.