The file which contains compiled versions of commonly used functions which can be linked to an object file to make an executable program.

Library files contain groups of often-used computer code that different programs can share. Programmers who use library code make their programs smaller since they do not need to include the code in their program. A virus that infects a library file automatically may appear to infect any program using the library file. In Windows systems, the most common library file is the Dynamic Link Library; its extension is DLL.