A coin is a flat, usually disc-shaped piece of metal, which is or has been used as a form of currency among some cultures, such as the Ferengi and Humans.
Some types of coins include:
Use as currency
While cleaning up his bar, Quark found three coins on the floor of the second level and shouted angrily to Broik and another Ferengi waiter that they should leave because of their carelessness. (DS9: "The Passenger")
In 2369, Nog paid with two coins for a jumja stick on Deep Space 9's Promenade. (DS9: "A Man Alone")
That same year, Quark narrowly avoided an assassination attempt as Grand Nagus by bending over to pick up a coin dropped on the floor of his bar. (DS9: "The Nagus")
In 2371, Quark gave Major Kira Nerys some latinum coins, good for five free spins at the dabo wheel, as one of the prizes she won as his "one millionth customer". (DS9: "Meridian")
Arcade games, such as Cardboard Chaos, were generally coin operated, though in some circumstances, tokens without monetary value could be used instead. (LD: "Starbase 80?!")
Other uses
Flipping a coin was a practice used to settle a dispute or make a choice between two options.
In 2368, when two different groups wished two use the thermal imaging array on the USS Enterprise-D, Lieutenant commander Data brought the problem to Lieutenant commander Geordi La Forge, who replied "Well, tell them to flip a coin", a reference to the practice of making a determination by which side of the coin was facing upwards when it landed. Data took La Forge literally and replied that he would replicate one. (TNG: "The Game")
In 2373, when overwhelmed with his decision to treat Harry Kim rather than Ahni Jetal, who ultimately died of her injuries, The Doctor questioned how he ought to choose between two dying people with an equal chance of survival; by flipping a coin or picking a playing card, perhaps. (VOY: "Latent Image")