WordNet (r) 3.0 (2006):

built-in
    adj 1: existing as an essential constituent or characteristic;
           "the Ptolemaic system with its built-in concept of
           periodicity"; "a constitutional inability to tell the
           truth" [syn: built-in, constitutional, inbuilt,
           inherent, integral]

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

built-in

   (Or "primitive") A built-in function or operator is one
   provided by the lowest level of a language implementation.
   This usually means it is not possible (or efficient) to
   express it in the language itself.  Typical examples are the
   basic arithmetic and Boolean operators (in C syntax: +, -,
   *, /, %, !, &&, ||), bit manipulation operators (~, &, |, ^)
   and I/O primitives.  Other common functions may be provided in
   libraries but are not built-in if they are written in the
   language being implemented.

   (1995-02-14)