A software tool that gives the programmer the ability to examine a program and watch it run. Sometimes to interact with it as it runs and change its environment.
Some of the things the programmer can do are these:

• Load a program
• Set a breakpoint
• Start the program
• Look at the code that is running
• Look at registers
• Examine memory (sometimes on a structured basis)
• Trace the flow
• Change the program sequence

Many times the debugger is part of a visual toolset that is included with the compiler and linker.