Wikipedia page for OpenAPI spec
The OpenAPI Specification, originally known as the Swagger Specification, is a specification for machine-readable interface files for describing, producing, consuming, and visualizing RESTful web services. Originally part of the Swagger framework, it became a separate project in 2016, overseen by the OpenAPI Initiative, an open source collaborative project of the Linux Foundation.
My favorite OpenAPI learning resources
In addition to the v2 Spec, I used the online Swagger Editor and followed these two tutorials to get an actual feel for what the Spec requires:
For the v3 OpenAPI Spec, the following resource looks promising as well:
Great beginner tutorial (read-along)
Create Your First OpenAPI Definition with Swagger Editor | BlazeMeter
OpenAPI definitions, formerly known as Swagger files, allow developers to specify the operations and metadata of their APIs in machine-readable form. This enables them to to automate various processes around the API lifecycle. We've already covered 5 reasons you should use OpenAPI/Swagger for your APIs on this blog.