Njord and his children joined the Æsir as Vanir hostages after the Æsir/Vanir war.
Njord is the Old Norse equivalent of the goddess Nerthus described by Tacitus. It has been suggested by Hilda R. Ellis Davidson in Gods and Myths of Northern Europe (1964) that there was possibly originally a male and female pair of deities, Njord and Nerthus, with Freyr later replacing Nerthus. She also makes the point that there were other male/female pairings of Norse gods of whom little is known but their names, e.g. Ullr and Ullin.
The comparative mythologist Georges Dumézil developed the idea introduced by Jacob Grimm that the hero Hadding in Saxo Grammaticus' Danish History, Book I, might be an euhemerized version of Njord. This suggestion was used by science-fiction/fantasy writer Poul Anderson in his War of the Gods.
Alternatives: Njord, Njörd, Njördh, Njörðr, (most accurate) Njǫrðr
In Viktor Rydberg's idiosyncratic and generally unaccepted reconstruction of Norse mythology Njord was also known as "Fridlief", the Lover of Frith. With Hodur, he undertook a mission of peace to Weland and Egil, which they refused. He rescued his son Freyr from the giants later on. During the war between Æsir and Vanir, he led the attack on Asgard and won. While he was gone from Vanaheim, Loki tried to take over there, but Njord defeated him in battle and routed him.