Wiktionary
n. (context economics English) A transaction for a very small amount of money (a micropayment).
Wikipedia
Microtransaction is a business model where users can purchase virtual goods via micropayments. Microtransactions are often used in free-to-play games to provide a revenue source for the developers. While microtransactions are a staple of the mobile app market, they are also available on traditional computer platforms such as Valve's Steam platform.
Free-to-play games that include a microtransaction model are sometimes referred to as "freemium". "Pay-to-win" is sometimes used as a derogatory term to refer to games where paying for in-game items can give the player an advantage over other players, particularly if the items cannot be obtained by free means. The objective with a free-to-play microtransaction model is to get more players into the game and provide desirable items or features that players can purchase if they are interested in them - it is hoped that in the long term the profits from a microtransaction system will outweigh the profits from a one-time-purchase game.
Items and features available by microtransaction can range from cosmetic (such as decorative character attire) to functional (such as weapons and items). Some games allow players to purchase items that can be acquired through normal means, but some games include items that can only be obtained through microtransaction. Some developers ensure that only cosmetic items are available this way to keep gameplay fair.