No, this is not a reference to the HAL 9000 from Clarke's 2001: A Space Odyssey,
Quick Answer
HAL is an acronym for Hypertext Application Language and is designed to provide "...a set of conventions for expressing hyperlinks in either JSON or XML." HAL is an IANA registered media type that enables RESTful APIs to return JSON (or XML) content with hypertext (aka hypermedia) that can be easily interpreted by machine and human agents. Because it is a registered media type, developers can specify application/hal+json as the value for the Content-Type response header.
Digging Deeper
In our What is REST? article we introduced an important aspect of the Uniform Interface constraint known as HATEOAS. The concept of HATEOAS borrows the concept of hyperlinks in HTML formatted media (via the anchor tag) to provide self-documented state changes. The difference is that the HTML media defines this as standard which makes it possible for clients to interpret the hyperlinks automatically. Plain old JSON (as well as XML) does not define hypermedia in any way. This means any hypermedia included in a JSON object has no self-interpreted meaning. Clients cannot know the difference between a URI that is part of the data model (e.g., a property called companyWebSite) versus a URI that clients can use to navigate to another resource.
Clients have to be told how to tell the difference between normal text versus hypermedia text for it to be useful.
This is why media types registered with the IANA exist. This provides a standards based mechanism for clients to reference.
