Muda is Japanese for waste: any activity that consumes resources without creating value for the customer. Lean classically identifies 7 types of waste — transport, inventory, motion, waiting, overproduction, over-processing and defects — often supplemented by an 8th: unused talent.
Waste is rarely visible as waste. It hides as “the way we do things here”: buffers that mask imbalance, waiting time perceived as natural, inspection that compensates for unstable processes. That is why the hunt for waste starts by making the value stream visible.
Overproduction is considered the worst type of waste, because it creates all the others: more inventory, more transport, more handling and later discovery of defects.