An API integration is the connection between two or more applications, via their APIs, that lets those systems exchange data. API integrations power processes throughout many high-performing businesses that keep data in sync, enhance productivity, and drive revenue.
The term API integration refers to how two or more applications can be connected to each other via their APIs to perform some joint function...using the API layer of two or more applications to make them talk to each other.
One of the most well-known API integration examples for marketing and revenue professionals is the Marketo -> Salesforce sync, which combines API integration between the two tools, and also adds an extra layer of automation - a trigger that automatically updates data in both applications in response to updates on one side for certain data types.