A counter is usually a small cardboard square moved around on the map of a board wargame to represent relevant information or determine certain things.
The first wargame based on cardboard counters was War Tactics or Can Great Britain Be Invaded?
Traditional wargames typically have hundreds of counters (The Russian Campaign, 225; GI: Anvil of Victory, 856; Terrible Swift Sword, 2,000).
Dragon Magazine used to include counters printed on a cardstock centerfold for monthly games (especially by Tom Wham).
Although counters are typically square, some games use oblong rectangles as counters for individual ships, as in Flying Colors, or for land units in tactical-scale games when the designer wishes to emphasize the facing or linearity of the unit, as in the Great Battles of History series.