File with a .DYLIB extension contains a dynamic library of declarations and functions utilised within systems based on the Mach core. Libraries are loaded on demand in the moment they are referred to by an application or a system function, which enables optimised management of the operating memory.

DYLIB libraries are used in Mac OS X or NeXTSTEP – both based on the Mach core. The name of the files – Mach-O is related with the objects of the Mach core. The reference to a DYLIB library can occur during running of an application, and also in the process of creating the Apple Xcode software.