BridgeSupport files are XML files that describe the API symbols of frameworks or libraries that cannot be introspected at runtime. 

These are generally ANSI C symbols that are non-object-oriented items such as constants, enumerations, structures, and functions but can also include some additional information about classes, methods, and informal protocols.

BridgeSupport files are a major component of the Objective-C bridges (RubyCocoa, PyObjC) and languages (MacRuby) which permit Cocoa development.

BridgeSupport comes with pre-generated files for all the system frameworks, as well as a tool that allows you to generate files for 3rd party frameworks or libraries. The sources are covered under a BSD license.

BridgeSupport ships in Mac OS X since version 10.5, Leopard. This project hosts development versions of BridgeSupport. Developers are encouraged to download the sources and report problems.