The term 'Tumbling Surface' refers to a special type of surface used for gymnastics and other activities that involve tumbling. The crossword solution for this term is MAT, which is an abbreviation for 'matted surface'. Mats are typically made of foam or rubber and provide a soft, cushioned surface for athletes to perform their tumbling routines on. They are also used for safety purposes, as they help to reduce the risk of injury from falls.