noun: A timing belt, or cam belt (informal usage), is a part of an internal combustion engine that controls the timing of the engine's valves. The belt itself is one of man's most painstaking automotive repairs as it requires 5-8 hours to replace. A timing belt repair is one of the most unsatisfactory jobs because it does not show any sign of improvement after replacing. A common timing belt repair is $700 dollars at a mechanic and almost $1,000 at a dealership. A poor functioning timing belt can also be an excuse to hang with friends.