1. A small, flat bar that is pressed with the foot to operate a machine: The seamstress used a treadle on her old-fashioned sewing machine.

Treadles are normally used to cause rotary motions in various machines by people who use their feet to sustain the movements.

2. A row of metal spikes that are set on an angle of a spring within a plate that is laid across an entrance or an exit of a parking lot: Some treadles are used to prevent drivers, who park their cars in a facility, from being able to leave without paying their parking fees.