An Application Programming Interface, or API, is a set of programming instructions that allow software applications to interact with each other. In the world of online trading, an API can be used to create custom trading applications or connect to third-party financial data providers.APIs can also be used to automate trading strategies or even place trades on behalf of the user. In recent years, the use of APIs has become increasingly popular among traders as they offer a convenient way to access a wide range of trading and market data.