in the forest; among the trees
Often used in compound nouns such as 林間学校 (forest school) or 林間コース (forest course). As a standalone noun, it refers to being within or among trees.
The children learned about nature at the forest school.
I like walking along the path through the woods.
school camp; outdoor school
Compound of 林 (woods) and 間 (interval, space), literally 'among the woods'.