Literally "lock": a light, targeted engagement that stabilises the torso. The best known one is mula bandha at the pelvic floor.
Bandhas are the most mystified part of yoga, yet you already know the feeling: mula bandha at the pelvic floor is roughly the engagement that happens involuntarily when you cough. In ashtanga they run continuously as part of the breath and core work — the invisible layer alongside posture, breath and gaze that turns gymnastics into moving meditation. The classic mistake is overdoing it: maximum squeeze, belly sucked in, breath held. That blocks the breath, and a blocked breath is yoga's first warning sign. What you want is a soft, sustained baseline tension you can keep breathing evenly underneath. The moment your breath stalls you're just straining – and it stops being a bandha.