An API (Application Programming Interface) is a set of protocols that allow different software applications to communicate with one another. In the context of trading and finance, APIs are critical for enabling integration between trading platforms, data providers, order management systems (OMS), and execution venues. For example, a quantitative strategy may use APIs to retrieve real-time market data, execute trades, and update portfolios without human intervention. APIs are essential for enabling automation, enhancing the speed of execution, and integrating disparate systems into a cohesive trading infrastructure. Institutions, including prime brokers and execution venues, often provide APIs to clients for custom trading solutions.