A complementary currency is a currency or medium of exchange that is not necessarily a national currency, but that is thought of as supplementing or complementing national currencies.^{: 3 : 2} Complementary currencies are usually not legal tender and their use is based on agreement between the parties exchanging the currency. According to Jérôme Blanc of Laboratoire d'Économie de la Firme et des Institutions, complementary currencies aim to protect, stimulate or orientate the economy.^: 7 They may also be used to advance particular social, environmental, or political goals.^: 4

When speaking about complementary currencies, a number of overlapping and often interchangeable terms are in use: local or community currencies are complementary currencies used within a locality or other form of community (such as business-based or online communities); regional currencies are similar to local currencies, but are used within a larger geographical region; and sectoral currencies are complementary currencies used within a single economic sector, such as education or health care. Many private currencies are complementary currencies issued by private businesses or organizations. Other terms include alternative currency, auxiliary currency, and microcurrency. Mutual credit is a form of alternative currency, and thus any form of lending that does not go through the banking system can be considered a form of alternative currency. Barters are another type of alternative currency. These are actually exchange systems, which trade only items, without the use of any currency whatsoever. Finally, LETS is a special form of barter that trades points for items. One point stands for one worker-hour of work, and is thus a time-based currency.