The term 'Build a Tree House' can be summarized by the solution to the crossword clue 'NEST', which is a four letter word. Building a tree house involves constructing a structure in a tree, typically for recreational purposes. It is often built as a platform or shelter, and may include features such as ladders, railings, and windows. The tree house may also be used as a play area for children or a place to relax and enjoy nature.