What is Synesthesia in the novel, A Mango-Shaped Space?

Synesthesia is basically a condition in which two or more of a person's senses merge in a specific situation. For example, Mia sees colors and shapes when she hears sounds. Some with this condition also taste specific things when they hear or see something. For some, there is a combination of associations. The condition is real and the author includes information, including websites associated with the disorder, at the end of the book.