A pinch is a climbing hold — and the corresponding grip technique — where the climber grips the hold by pressing the thumb against the fingers from opposing sides. The hand acts as a clamp. Pinches range from narrow (thumb and one or two fingers, requiring high force) to wide (thumb and all four fingers spread, more like a sloper).
Pinching demands a specific kind of strength: thumb opposition strength, plus the muscles of the hand that bring the thumb across to meet the fingers. This is rarely trained directly outside climbing — most non-climbers have weak pinch strength relative to their crimp strength because daily life gives the thumb little opposition work.
Pinches tend to appear on overhanging routes and modern bouldering setups. Indoor route-setters use them to break up otherwise edge-heavy sequences. To improve pinch strength, do hangboard pinch holds, plate pinches with weights, and just climb on more pinch-heavy routes — the body adapts surprisingly fast.