Hex mat

hex mat, n. a plastic sheet ruled with lines forming an hexagonal grid, and designed to be written on with erasable ink pen; commonly used in RPGs and wargames for drawing maps; also battlemat. See also hex, hex paper. [GJ1]
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Addendum: Most commonly, maps are drawn on the hex mat at the same 25mm scale as lead miniatures; the mat is placed flat on a table; then the miniatures are placed on the mat, and moved around as game play ensues. [GJ1]
Addendum #2: Modern battlemats are usually drawn with squares instead of hexes to accommodate systems such as D&D post 3rd Edition , though some are reversible with squares on one side and hexes on the other.