The Collaborative International Dictionary of English v.0.48:

Portability \Port`a*bil"i*ty\, n.
   The quality or state of being portable; fitness to be
   carried.
   [1913 Webster]

WordNet (r) 3.0 (2006):

portability
    n 1: the quality of being light enough to be carried

The Free On-line Dictionary of Computing (30 December 2018):

portability
portable

    The ease with which a piece of
   software (or file format) can be "ported", i.e. made to run
   on a new platform and/or compile with a new compiler.

   The most important factor is the language in which the
   software is written and the most portable language is almost
   certainly C (though see Vaxocentrism for counterexamples).
   This is true in the sense that C compilers are available for
   most systems and are often the first compiler provided for a
   new system.  This has led several compiler writers to compile
   other languages to C code in order to benefit from its
   portability (as well as the quality of compilers available for
   it).

   The least portable type of language is obviously assembly
   code since it is specific to one particular (family of)
   processor(s).  It may be possible to translate mechanically
   from one assembly code (or even machine code) into another
   but this is not really portability.  At the other end of the
   scale would come interpreted or semi-compiled languages
   such as LISP or Java which rely on the availability of a
   portable interpreter or virtual machine written in a lower
   level language (often C for the reasons outlined above).

   The act or result of porting a program is called a "port".
   E.g. "I've nearly finished the Pentium port of my big bang
   simulation."

   Portability is also an attribute of file formats and depends
   on their adherence to standards (e.g. ISO 8859) or the
   availability of the relevant "viewing" software for different
   platforms (e.g. PDF).

   (1997-06-18)