A system file is any file with the system attribute
turned on. Most Windows computers are configured by default not to display system files in normal file searches or in folder views.
The most popular system files you might encounter on a Windows computer include msdos.sys, io.sys, ntdetect.com and ntldr.
Examples: "I was concerned during a troubleshooting process that I was missing a number of system files. I finally realized that I had to tell Windows to show those files since they're hidden from view by default. It turns out I wasn't missing anything at all."