What is the X in X-mas?
(Or "copy member", "copy module") A common piece of source code designed to be copied into many source programs, used mainly in IBM DOS mainframe programming.
In mainframe DOS (DOS/VS, DOS/VSE, etc.), the copybook was stored as a "book" in a source library. A library was comprised of "books", prefixed with a letter designating the language, e.g., A.name for Assembler, C.name for Cobol, etc., because DOS didn't support multiple libraries, private libraries, or anything. This term is commonly used by COBOL programmers but is supported by most mainframe languages. The IBM OS series did not use the term "copybook", instead it referred to such files as "libraries" implemented as "partitioned data sets" or PDS.
Copybooks are functionally equivalent to C and C++ include files.