{"API Definitions"}

Some Organizations To Watch

These are the companies, and organizations who are doing interesting things when it comes to defining APIs. If they have an API, I'm working to profile, and provide machine readable indexes of each one below. If they do not have an API, I will just provide a brief profile of their operations.

Apache Avro
Apache Avro is a data serialization system. Avro relies on schemas. When Avro data is read, the schema used when writing it is always present. This permits each datum to be written with no per-value overheads, making serialization both fast and small. This also facilitates use with dynamic, scripting languages, since data, together with its schema, is fully self-describing.
API Blueprint
API Blueprint is a documentation-oriented API description language. A couple of semantic assumptions over the plain Markdown. API Blueprint is perfect for designing your Web API and its comprehensive documentation but also for quick prototyping and collaboration. It is easy to learn and even easier to read – after all it is just a form of plain text.
API Transformer
Transform API Descriptions from/to various formats e.g., Swagger, API Blueprint, RAML, WADL, Google Discovery, I/O Docs. Please contact us for production and high volume usage.
Elastic Path
Elastic Path built Helix because existing API modeling languages, such as RAML, Swagger, and API Blueprint, while good for describing standard APIs, are not well suited to designing hypermedia APIs. Elastic Path Cortex, our patented API technology, utilizes Helix definitions and a Helix-compatible implementation, allowing our partners and customers to extend existing APIs and quickly create new ones.
You can use the Google API Discovery Service to build client libraries, IDE plugins, and other tools that interact with Google APIs. It provides a lightweight, JSON-based API that exposes machine-readable metadata about Google APIs, but could also be used to describe other APIs.
OData (Open Data Protocol) is an OASIS standard that defines the best practice for building and consuming RESTful APIs. OData helps you focus on your business logic while building RESTful APIs without having to worry about the approaches to define request and response headers, status codes, HTTP methods, URL conventions, media types, payload formats and query options etc.
OpenAPI Spec
The Open API Initiative (OAI) was created by a consortium of forward-looking industry experts who recognize the immense value of standardizing on how REST APIs are described. As an open governance structure under the Linux Foundation, the OAI is focused on creating, evolving and promoting a vendor neutral description format. SmartBear Software is donating the Swagger Specification directly to the OAI as the basis of this Open Specification.  
Postman on Chrome is the most efficient way to test, develop and document APIs. Create complex requests, go back in time and view results in a beautiful way. Postman has more than 80 features to help you at every step.
RESTful API Modeling Language (RAML) is a simple and succinct way of describing practically-RESTful APIs. It encourages reuse, enables discovery and pattern-sharing, and aims for merit-based emergence of best practices. The goal is to help our current API ecosystem by solving immediate problems and then encourage better API patterns. RAML is built on broadly-used standards such as YAML and JSON and is a non-proprietary, vendor-neutral open spec.
Semantic descriptions for hypermedia APIs. RESTdesc allows you to capture the functionality of hypermedia APIs, so automated agents can use them. Despite their powerful capabilities, RESTdesc descriptions are easy to master. Description is not a goal in itself: you want your API to be used. See how RESTdesc opens up your API for discovery, based on its functional characteristics.
Schema.org is a collaborative, community activity with a mission to create, maintain, and promote schemas for structured data on the Internet, on web pages, in email messages, and beyond. Schema.org vocabulary can be used with many different encodings, including RDFa, Microdata and JSON-LD.
U.S. Data Federation
The U.S. Data Federation supports data interoperability and harmonization across Federal, state, and local government agencies by highlighting common data formats, API specifications, and metadata vocabularies.

If there is an organization you think should be listed here, you can submit as Github issue for this research project and I will consider adding.