A name in the Sindarin tongue for one of the six seasons that made up a year for the Elves. The name Narbeleth means 'Sun-waning', and refers to a period of fifty-four days that saw the fading of the year into winter (for this reason Narbeleth is commonly called Firith, 'fading', or Quellë in Quenya). Within the calendar of the Elves Narbeleth came between the Enderi, the three middle-days of the year, and the long season of Rhîw or winter. On a modern calendar Narbeleth ran between 28 September and 20 November.

Perhaps confusingly, the name Narbeleth also appears as the name of the tenth month in the calendars of the Dúnedain. The two are related in the sense that they occur in late autumn, but they are not equivalent. The Narbeleth in the calendars of Men covered only approximately the first half of the Elves' Narbeleth, coming to an end on modern 21 October.