Definition of coasters:
(n) :
(by extension) A small, flat or tray-like object on which a bottle, cup, glass, mug, etc., is placed to protect a table surface from drink spills, heat, or water condensation.
(n) :
(dated) A small stand or tray, sometimes with wheels, used to pass something such as a decanter or wine bottle around a tabletop.
(n) :
(nautical) A merchant vessel that stays in coastal waters, especially one that travels between ports of the same country.
(n) :
A person who originates from or inhabits a coastal area.
(n) :
(nautical) A sailor (especially the master or pilot of a vessel) who travels only in coastal waters.
(n) :
(US, winter sports) A person who uses a sled or toboggan to slide down a slope covered with ice or snow; a sledder, a tobogganist.