The Free On-line Dictionary of Computing (30 December 2018):
debugging by printf
The debugging technique where the programmer
inserts print statements into a program so that when run the
program leaves a "trail of breadcrumbs" allowing him to see
which parts were executed. The information output may just be
a short string to indicate that a particular point in the code
has been reached or it might be a complete stack trace. The
output typically just goes to the window or terminal in which
the program is running or may be written to a log file.
printf is the standard C print function, other languages
would use different names.
(2007-03-08)