The hidden valley within the Echoriath where Gondolin was built (S/115), translated “Wide Valley” (SI/Tumladen). It was also the name of a valley in Gondor (LotR/764) glossed as a “flat valley with steep sides” (RC/521). This name is a combination of tum “valley” (SA/tum) and laden “open, cleared”.
Conceptual Development: In the earliest Lost Tales, this valley was named G. Tumladin with an i and the gloss “Valley of Smoothness” (LT2/163), a form that continued to appear in Silmarillion drafts from the 1930s (SM/148, LR/142). In The Etymologies from the 1930s, it appeared as N. Tumladen with an e and the gloss “Level Vale”, with essentially the same derivation given above (Ety/LAT, TUB). This later form was adopted in Silmarillion revisions from the 1950s-60s (WJ/57). In one late note from around 1968, Tolkien gave the name as Tumlaðen (NM/351).
tum (“deep valley”) + laden (“clear, open, wide”)