启动磁区是硬盘、软碟或类似的数据保存设备的一个磁区，内含负责启动（booting）「存放在盘片（disk）的其他部份的程序（通常，但不必然是操作系统）」的机械码。

引导扇区有两种：

IBM PC兼容机上，BIOS不在意VBR与MBR的不同，甚至分割区。韧体只是加载并运行磁盘的第一个磁区(sector)，在MBR里的码，才知道磁盘分割消息；且是负责加载启动活动分割区的VBR的地方。

如果你从「一个没有安装操作系统的磁盘」启动，屏幕会显示"Please Insert a bootable disc and press a key"；这是开机磁区(boot sector)显示的，而不是机器的韧体。

A boot sector or boot block is a region of a hard disk, floppy disk, optical disc, or other data storage device that contains machine code to be loaded into random-access memory (RAM) by a computer system's built-in firmware. The purpose of a boot sector is to allow the boot process of a computer to load a program (usually, but not necessarily, an operating system) stored on the same storage device. The location and size of the boot sector (perhaps corresponding to a logical disk sector) is specified by the design of the computing platform.