IntelĀ® MPI Library Reference Manual for Linux* OS
The term ILP64 means that integer, long, and pointer data entities all occupy 8 bytes. This differs from the more conventional LP64 model in which only long and pointer data entities occupy 8 bytes while integer entities occupy 4 bytes. More information on the historical background and the programming model philosophy can be found, for example, in http://www.unix.org/version2/whatsnew/lp64_wp.html