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

debugger
debugging tool

    A tool used by a programmer to monitor
   and control a program he is trying to fix.  The most important
   functions of a debugger are tracing, stepping, breakpoints and
   watches.

   Tracing displays a step-by-step report on what statement the
   program is currently executing, allowing the programmer to follow
   the flow of control through if statements, loops (loop),
   subroutine calls, etc.

   Breakpoints and watches both pause execution of the program
   and return control to the debugger under certain conditions.  A
   breakpoint triggers when execution reaches a particular
   statement in the program and a watch triggers whenever a
   specific variable is modified.  Stepping is like a breakpoint on
   every statement, often with the option to step "into" or "over" a
   subroutine, i.e. continue stepping through the statements of the
   subroutine or just execute it without pausing and resume stepping
   when it returns.

   Whenever control returns to the debugger it lets the programmer
   ask to see the values of variables, and possibly modify them,
   before resuming execution.  Some debuggers can be set to
   automatically perform some action like display a variable value
   and resume.

   A debugger can interact with the target program in different ways.
   Some debuggers require the program to be loaded into the debugger
   which may then modify or "instrument" the program for debugging.
   Others can "attach" to a program that is already running.  Some
   are built into the normal program execution environment (e.g. an
   interpreter) and can be set to run under certain conditions,
   e.g. errors.

   Early debuggers such as Unix's adb only knew about the
   compiled executable code so sometimes debugging had to be done at
   the level of machine code instructions and numerical memory
   locations.  If you were lucky, the debugger could access the
   program's symbol table and display the original names of
   subroutines and variables.  Sometimes this required the program to
   be "compiled for debugging".  Since compiling every program for
   debugging would add significantly to the size of a distribution
   of a whole operating system, it is common for programs to be
   distributed without debugging support but for individual programs
   to be made available with it.

   A major advance in debuggers was source-level debugging.  This
   gives the programmer a view of their source code annotated with
   breakpoints and a pointer to the statement currently being
   executed.  Such a view is commonly part of an integrated
   development environment like Visual Basic.

   (2014-08-23)