The Collaborative International Dictionary of English v.0.48:

Extensional \Ex*ten"sion*al\, a.
   Having great extent.
   [1913 Webster]

WordNet (r) 3.0 (2006):

extensional
    adj 1: defining a word by listing the class of entities to which
           the word correctly applies

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

extensional

   Extensional properties, e.g. extensional equality, relate to
   the "black-box" behaviour of an object, i.e. how its output
   depends on its input.  The opposite is intensional which
   concerns how the object is implemented.