A PC booter, or booter, is a type of software for home computer era (early 1980s to early 1990s) personal computers that was loaded and executed in the bootup of the computer, from a bootable floppy disk, rather than as a regular program; a booter thus bypassed any operating system that might be installed on the hard disk of the PC. Games were the type of software most commonly distributed as booters.

Reasons for preferring booters to standard programs include ease of use (the software would start automatically, without any further action required by the user), reliability (few chances to manually alter program files), copy prevention (the booter floppies can be hard to read with a regular operating system and might even have a nonstandard filesystem or formatting), and avoiding a normal operating system (to spare some space on the floppy or to use some specialized replacement).

Source: Wikipedia, "PC booter," available under the CC-BY-SA License.