The mallorn (plural mellyrn - the word is Sindarin) is a kind of large tree.
Mellyrn are described as similar to birches: their bark was smooth and silver-grey, and the leaves turned golden in autumn. The golden leaves remained on the tree through the winter and fell to cover the ground in the spring, when new leaves sprouted, which were green on top and silver underneath, and golden flowers bloomed on the branches. Frodo Baggins explained that he could feel the life of the tree when he touched a mallorn in Lothlórien.
Mallorn-trees originally grew on Tol Eressëa, and probably also grew in Valinor. The Noldor may have introduced mellyrn to Gondolin when they came to Middle-earth, but these were all destroyed with the city. The Elves of Tol Eressëa brought mallorn-trees to the Men of Númenor and they grew on the shores around the Bay of Eldanna in that land. Even Tar-Aldarion, the great Ship-King, did not cut down these trees.
Mellyrn-nuts were given by Tar-Aldarion to his friend Gil-galad, the Noldorin King of Lindon. The mellyrn did not grow in Lindon, but Galadriel took some nuts with her to Lothlórien, where they grew to immense heights. Lothlórien became known as the Golden Wood because of them.
Caras Galadhon, the city of Galadriel and Celeborn in Lothlórien, was built in the branches of huge mallorn-trees. The Fellowship spent the night in a flet (a sort of platform) in a mallorn-tree nearby, and were later given lembas wrapped in mallorn-leaves.
Galadriel gave Samwise Gamgee a box of soil containing a single silver mallorn nut. After the War of the Ring, Sam planted the nut in the Shire, in the Party Field where the Party Tree had stood. It was the only mallorn-tree in Middle-earth outside of Lórien. When the tree bloomed in the next summer it was said that all the Shire became golden from the flowers.