A loop of code that executes without releasing any resources to other programs or the operating system.
Consider the following pointless BASIC loop that counts upward indefinitely
10 i = i + 1
20 GOTO 10
Run on a single-user system such as MS-DOS this will not cause any problems. Run on a cooperative multitasking operating system such as Windows 3, the system would appear to freeze.
A pre-emptive multitasking operating system such as UNIX or Windows NT would "steal" cycles away from the program and continue to run other programs.
See also busy-wait and multitasking.