Poka-yoke (mistake-proofing) is any mechanism that makes errors impossible or immediately obvious — asymmetric connectors that only fit one way, sensors that halt a machine if a step is skipped, fixtures that won't accept a misoriented part.
It embodies the lean belief that defects come from process design, not careless people. Poka-yoke is a primary countermeasure generated by FMEA and a core element of jidoka.