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).
Some booters include a customized subset or variant of a "standard" operating system for the platform (for example, DOS for IBM PC compatible, Apple DOS or ProDOS for Apple II, etc.).
Today, IBM PC compatible computers can still boot from floppies, CD-ROMs and DVDs, USB storage devices etc, but the computer's BIOS is usually set to boot from hard disk.
Amiga games were often distributed as bootable floppies using a custom boot block which would consist of a custom loader. These disks contained no filesystem; instead, the custom loader would read the tracks directly. Many Amiga games were released as such in order to thwart piracy, and to utilise the RAM otherwise occupied by the AmigaOS. In early to mid-1990s, disks with a custom boot block became very popular for making so called trackmos by demo groups.
While booters provided a safe form of copy protection, programs such as Locksmith and Copy II PC existed that provided a method for copying of these disks; these were known as nibble copiers.