A "Linker Error" occurs during the process of combining multiple object files into a single executable program. This error typically arises when the linker cannot find the definitions for functions or variables that were declared but not defined in the code. It indicates that there are unresolved references, which means the program cannot be successfully compiled and run. Common causes of linker errors include missing libraries, incorrect file paths, or mismatched function signatures. Developers often encounter these errors when working with programming languages like C or C++, where separate compilation of code files is common. Fixing linker errors usually involves ensuring all necessary files and libraries are correctly included.