"Caster" refers to a wheel or a device used to make objects move easily, typically by attaching it to the bottom of furniture or equipment. It allows the object to roll or swivel smoothly in any direction. Full definition
All eyes were on Caster Semenya on Saturday afternoon. (sbnation.com)
This all helps make the magic system feel like an active part of battles and not just something that you see happen in the background; the spell casters of your group all feel more essential as a result. (snackbar-games.com)