The term "caster" has several meanings depending on its context.
1) In a general sense, it can refer to someone who casts something such as a vote or an object like a fishing line.
2) In sports terminology, particularly in reference to wheelchairs, caster is the angle of inclination between the vertical and the groundline of the wheelchair user's footrests. This can affect how quickly the chair moves forward when pushed by the wheels or the user's feet.
3) In a more specific context related to bicycles, it refers to the mechanism that allows the front wheel to turn in response to steering input from the rider. The caster is attached to the fork and includes bearings that allow for smooth rotation of the wheel.