What is an Object File?

An object file is basically a file containing machine language instructions and data in a form that the linker can use to create an executable program. Each routine or data item defined in an object file has a corresponding symbol name by which it is referenced. A symbol generated for a routine or data definition can be either a local definition or global definition. Any reference to a symbol outside the object file is known as an external reference.

explanations of Local Definitions, Global Definitions, External References follow

from http://docs.hp.com/en/B2355-90655/ch02s03.html