The Free On-line Dictionary of Computing (18 March 2015):
The process of attempting to determine the cause of
the symptoms of malfunctions in a program or other system. These
symptoms may be detected during testing or use by real users.
Symptoms are often caused by factors outside the program, such as
misconfiguration of the user's operating system,
misunderstanding by the user (see PEBCAK) or failures in other
external systems on which the program relies. Some of these are
more in the realm of technical support but need to be
eliminated. Debugging really starts when it has been established
that the program is not behaving according to its specification
(which may be formal or informal). It can be done by visual
inspection of the source code, debugging by printf or using a
debugger. The result may be that the program is actually
behaving as specified but that the spec is wrong or the
requirements on which it was based were deficient in some way (see
Once a bug has been identified and a fix applied, the program must
be tested to determine whether the bug is really fixed and what
effects the changes have had on other aspects of the program's
operation (see regression testing).
The term is said to have been coined by Grace Hopper, based on
the term "bug".