Yith is a fictional planet, an earlier home of the Great Race of Yith, a prominent species within the Cthulhu Mythos. (HPL: The Shadow Out of Time) The name first appeared in Duane W. Rimel's poem cycle, "Dreams of Yith"; Rimel had originally written "Yid", but Lovecraft changed that when revising the poems, noting that Rimel's word was also a derogatory word for Jews.
The word "Yith" appears only once in Lovecraft's fiction, when he notes in The Shadow Out of Time that "the minds of the Great Race sped across the void from that obscure trans-galactic world known in the disturbing and debatable Eltdown Shards as Yith."