The Collaborative International Dictionary of English v.0.48:

Extensible \Ex*ten"si*ble\, a. [Cf. F. extensible. See
   Extend.]
   Capable of being extended, whether in length or breadth;
   susceptible of enlargement; extensible; extendible; -- the
   opposite of contractible or compressible. "An extensible
   membrane" --Holder.
   [1913 Webster]

WordNet (r) 3.0 (2006):

extensible
    adj 1: capable of being protruded or stretched or opened out;
           "an extensile tongue"; "an extensible measuring rule"
           [syn: extensile, extensible] [ant: inextensible,
           nonextensile, nonprotractile]

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

extensible

    Said of a system (e.g., program, file
   format, programming language, protocol, etc.) designed to
   easily allow the addition of new features at a later date,
   e.g. through the use of hooks, an API or plug-ins.

   See also extend, forward compatible.

   (1998-01-15)