A special character to signify that this is the end of the current input stream. A file is just another way of saying input stream.
EOF Value
It is CTRL+D in linux systems
it is CTRL+Z in windows
EOF is either -1 or 0. In newer C language, EOF is set as -1. stdio.h automatically defines EOF as a C Macros
https://stackoverflow.com/questions/1782080/what-is-eof-in-the-c-programming-language
EOF Use-case
Input streams may switch. Every input stream is prefaced with a CTRL+D character to signify the input stream has switched.
You may want to check for these if you want to check for input stream switches.