The word portability has many meanings when used with respect to computer programs. Here, I interpret it to be the amount of modification which must be made to a program before it will run natively on a system other than that on which is was written. The original aim of the GNU system was to have a system which would be source-code compatible between many varying machines. Sun's Java system aims to be binary-compatible between systems, but the Free Software Foundation do not have this aim since they believe that source code should be available for all programs.