Host features are those which the implementation adds around the native feature set. Host features would include new object types. There are entire groups of objects which collected together provide a host environment capability.

For example, the HTML Document Object Model describes a page as it lives inside a web browser.

The PDF file format can contain a collection of objects and the PDF reader adds others to construct a different kind of object model.

A process control system might contain an object model representation of a brewery or a nuclear reactor, although it is unlikely you would control either with just a JavaScript interpreter. You could build a monitoring system with a mimic display that was a fairly good illustration of the processes though.

The browser, server or operating system is said to host the interpreter. That is where the terminology of host environments, host features and so on is derived.

Cross-references:

Wrox Instant JavaScript - page - 12

Wrox Instant JavaScript - page - 42