What is the name of the room in Riley's mind where things become abstract?

The room in Riley’s mind where things become abstract is called Abstract Thought.

Abstract Thought is one of the imaginative regions inside Riley’s mind. When Joy, Sadness, and Bing Bong enter it, their forms progressively lose detail and become more simplified. The sequence turns an idea about abstraction into a visual gag: the characters become two-dimensional, then increasingly fragmented and difficult to recognize.

The area appears while Joy and Sadness are traveling through Riley’s long-term memory. Bing Bong accidentally leads them into Abstract Thought, causing them to miss the Train of Thought. Their detour illustrates how Riley’s imagination and mental processes are represented as literal places in the film.

Abstract Thought is not the same as Imagination Land, where Riley’s fictional creations are built, or Dream Productions, where her dreams are staged. It is also distinct from the Memory Dump, which is where forgotten memories disappear.