Aldalómë is one of the names of the Fangorn Forest, used by Treebeard in his song In the willow-meads of Tasarinan I walked in the Spring (LoTR II, Ch. 4). The other names mentioned by Treebeard are Ambaróna and Tauremorna.
J.R.R. Tolkien notes in his Unfinished Index that it means 'tree-twilight', and refers to Quenya words alda 'tree' and lómë 'dimness, twilight, night' (quoted in Scull - Hammond, Reader's Companion, p. 385).