A pass through the mountains surrounding Gondolin, translated “Eagles’ Cleft” (S/243). This name is a combination of cirith “cleft, ravine” and the class plural of thoron “eagle” (SA/kir, thoron).
Conceptual Development: In the earliest Lost Tales, this pass was called G. Cristhorn (LT2/191), a combination of G. cris(s) “cleft” and G. thorn “eagle” (GL/27, 73). In Silmarillion drafts from the 1930s, the name generally appeared as N. Cristhorn (SM/308, LR/142), but in the period Tolkien also considered various alternatives such as N. Cristhoron with N. thoron “eagle” (Ety/KIRIS), Cilthoron(dor) with N. cîl “cleft” (Ety/KIL) and in one place an early appearance of its later name Kirith-thoronath (SM/146).
The name was revised to Cirith Thoronath in the texts used for the published version of The Silmarillion, but the earlier form (Cristhorn) appeared in a late essay on Glorfindel (PM/379), so perhaps Tolkien had ongoing uncertainty about this name.
cirith (“pass, cleft”), thoron (“eagle”) + ath (collective plural suffix)