Mach-O

from Wikipedia:

Mach-O, short for Mach object file format, is a file format for executables, object code, shared libraries, dynamically-loaded code, and core dumps. A derivation of the a.out format, Mach-O offered more extensibility and faster access to information in the symbol table.

Mach-O was once used by most systems based on the Mach kernel. NeXTSTEP, Darwin and Mac OS X are examples of systems that have used this format for native executables, libraries and object code. GNU Hurd, which uses GNU Mach as its microkernel, uses ELF, and not Mach-O, as its standard binary format.

File Format Reference can be found here