A treehouse is a structure built in the branches of a tree, usually for recreational purposes. The term 'treehouse' can also be used to refer to a playhouse or a small shelter built for children. The crossword solution for the term 'treehouse' is 'NEST, ' which is a four-letter word that refers to a structure built by birds for nesting and raising their young. The term 'NEST' is often used metaphorically to refer to a safe and comfortable place, which is similar to the purpose of a treehouse.