The Celos was a lesser river or stream that flowed out of the White Mountains into the Gondorian land of Lebennin, a little to the west of Minas Tirith. It was a tributary of the larger River Sirith.
Location
The smaller-scale map that accompanies The Lord of the Rings and Unfinished Tales shows the Celos flowing into the Sirith from the east, while the large-scale contour map of Gondor shows it flowing from the west. A brief reference in Unfinished Tales suggests that the Celos was the next river eastward from the Gilrain, which would make the large-scale map correct. The mistake was carried over to The West of Middle-earth at the End of the Third Age for the Unfinished Tales. Christopher Tolkien noted the error of the small-scale map in The Treason of Isengard; the correct location of the Celos is thus being the western tributary.
Later prints of the Unfinished Tales (but not all) corrected the labels.