A Synesthesia in NLP is an automatic link from one sense to another. Eg Seeing something in your mind and getting a feeling associated with that picture simultaneously.
This is part of NLP Strategies, understanding how we make decisions. You can learn how other people make decisions by understanding their strategies.
Think of the power of this in Sales, Education and Therapy! We offer full training in this and all areas of NLP on our NLP Training Courses.