Muda is the Japanese word for waste — any activity that absorbs resources without adding value the customer would pay for. The classic seven wastes (extended to the 8 wastes) are overproduction, waiting, transport, over-processing, inventory, motion, defects, and unused talent.
Muda is one of the "3 Ms" of waste alongside mura (unevenness) and muri (overburden) — and the latter two are often the upstream causes of the former.