This is the stage of the API lifecycle where you define the foundation for the business and technical details of your APIs, and where the work will occur to produce and iterate upon the API. The more work done here during the definition stage, the more time you will save during other stages, however, you won’t always be able to define everything and will need to get to work designing and developing at some point.
The define stage should setup source control and plant the seeds of the variety of artifacts that are needed to define the business and technical details of the API and the operations around it. The source control and artifacts will make up the base contract and manifest for everything that happens across each stage of the lifecycle. Product and engineering stakeholders should work together to define each API the best they can to lay the right foundation.