A turnstile is the gate at an entrance that spins, with arms like the spokes of a wheel, letting exactly one person through at a time. You meet them at MRT and subway stations, at stadiums, at museums, at swimming pools — anywhere a place needs to check tickets and count heads. The counting matters: because only one body fits between the arms, the machine knows how many people came in, which is why sports reports talk about turnstile numbers meaning attendance. Some are waist-high with three arms; the tall ones that go from floor to ceiling, often at the back gates of a stadium or a factory, get called full-height turnstiles. The word is ordinary and plain — nobody thinks of it as difficult English — and it has spread a little into computers, where a turnstile can mean any gate that lets requests through one by one.