noun, noun or participle which takes 'suru', intransitive verb
yellowing autumn leaves; golden fall foliage
Specifically refers to leaves turning yellow in autumn, as opposed to 紅葉 (red leaves). Can be used as a noun or as a suru-verb for the process of leaves turning yellow.
See also: 紅葉
イチョウの黄葉が美しい季節になった。
It's the season when the ginkgo trees' yellow leaves are beautiful.
山
The trees on the mountain have started to turn yellow.