An H file is a header file referenced by a document written in C, C++, or Objective-C source code. It may contain variables, constants, and functions that are used by other files within a programming project. H files allow commonly used functions to be written only once and referenced by other source files when needed.

H header files are typically used to store C function declarations, as well as macro definitions, which can be used by multiple source files. Header files can be inserted into a .C source code file using the #include directive. NOTE: H files may also store headers for C++ source code, but those headers are typically saved as .HPP files.