Final answer:
A JCL data definition statement associates a program-coded file name, known as a DDNAME, with an actual dataset on the mainframe. It is key for mapping files within a JCL job.
Step-by-step explanation:
The question you're asking about relates to JCL (Job Control Language), which is used on IBM mainframe systems to instruct the system on how to run a batch job or start a subsystem. Specifically, the question pertains to DD (Data Definition) statements in JCL, which are crucial for mapping files to programs within a JCL job.
A JCL data definition statement, often abbreviated as DD, serves to associate a program-coded file name, also known as a DDNAME, with an actual dataset on the mainframe. This step is vital because it tells the JCL job and the running program where to get input data from or send output data to. A DD statement might look something like this in JCL:
//DDNAME DD DSN=dataset.name,DISP=SHR
In this example, 'DDNAME' would be the name referenced in the program code, and 'dataset.name' is the actual name of the dataset on the mainframe system.