Nuada Airgetlám was the chief god of the Tuatha Dé Danann, the gods of Irish mythology. He was the god of healing, the Sun, childbirth, youth, beauty, ocean, dogs, poetry, writing, sorcery, magic, weapons, and warfare*, although he was not necessarily the head god in these areas. He married Macha, and fathered Tadhg and Echtghe (AKA Aughty). Both Nuada and Macha are slain by Balor. Nuada is also known as Nudd or Ludd.
Nuada was wounded in the first battle of Moy Tura, fighting the champion of the Fir Bolg** (the original inhabitants of Ireland). During this battle he lost his right arm; as a king of the Tuatha Dé, he would not be allowed to rule if he was not physically perfect, so Dian-cécht, the God of healing, fashioned him a silver hand. In some versions of the story the silver hand was sufficient to allow Nuada to return to kingship, but in other versions he had to wait until Airmed and Miach healed his arm completely.
While the business with his arm was getting sorted out, the evil Fomorian prince Bres took over as king, but he was tyrannical and unpopular; he was exiled as soon as possible, and Nuada replaced him, new arm in place. Unfortunately, Bres did not take his exile kindly, and started a war with the Tuatha Dé; during this war Nuada was killed, but the Tuatha Dé persevered, and Nuada's chosen successor, Lugh, took the throne.
It is generally accepted that Elcmar and Nechtan are also names for Nuada.
* Thanks to http://www.pantheon.org/articles/n/nuada.html for this list, where-ever they got it.
** Nuada and his army finally did win the war, and granted the Fir Bolg champion, Streng, and his followers one-quarter of Ireland; they chose Connacht as their land.